The Rise of Slack and The Death of Skype

by Time Doctor

Will Slack ever defeat Skype? We counted the odds.

According to and Forbes, Slack is one of the fastest growing startups, boasting more than 5 million daily users and 100 million website visitors per month.

Because of Slack’s meteoric success, we started to wonder: Will it ever replace Skype as the communication leader among businesses?

At Time Doctor, we have the fortune to anonymously track which apps and websites users visit throughout the work day. In other words, we can track how much time a user spends on Slack vs. Skype. This gives us the insight we need to determine if (or when) Slack can overtake Skype as the de facto communication tool among remote businesses.

Spoiler alert - it did not… yet.

However, our analysis shows that if all trends stay the same, Slack will inevitably catch Skype in about two years. Although Skype is still more widely used than Slack, our data confirms that Slack is gaining fast.

For this story we peeked into general stats, looked at the names of the app that people worked on, how much time they spent on the app, and when they logged in and out. With this data, we are able to see how much time Time Doctor users spend on Slack versus Skype to communicate with their colleagues.

Slackin’ away

Before we go into comparing Slack to Skype, let’s first get an understanding of the data. This first chart here shows us how much time per day all TimeDoctor users spent on Slack in 2016 and the first half of 2017.

From this graphic we can see that daily Slack usage varied from around 20 hours on some weekends close to 1000 hours onworkdays. You are welcome to explore the chart yourself - feel free to interact with the timeline and look around.

The chart is “spiky” for a simple reason, which is also true for most other apps TimeDoctor users work on - much less work time is logged during weekends. As a matter of fact, on average, Slack was used 9.4 times less on a weekend then over the week (an average of 771.6 hours logged for Slack across Time Doctor on a weekday versus 82 hours on a weekend).

The outliers

In the context of any data set, outliers are the data points that just don’t fit the overall pattern. Sometimes those are errors in data or false negatives. In our case, use of Slack and Skype during the weekends are such outliers.


In our analysis and comparison, we decided to exclude data collected during weekends so they don’t skew averages or influence the overall picture. Also, we only accounted for cases when a user logged between 1 minute and 10 hours per one particular task. This helped exclude records of 10 or 20 seconds or several that were 20 and more hours long - those were either very rare outliers or generally didn’t help us much in doing the overall analysis. Simply put, this just made the picture clearer.

In 2016, since January 2nd and 3rd was a weekend, this chart begins on January 4th. Though the picture is more clear now, there are still many spikes in the data, many of which are attributed to holidays.

TimeDoctor users logged time from countries all across the globe. Since we can’t really filter for all the holidays across all countries and cultures (that would just mean too many dates excluded), we decided to exclude several days that were way off, behaved and looked like outliers.

Those days are biggest spikes, clearly seen in the chart now. Let’s zoom into it for a minute so we could have a better look.


We are now looking at the summer 2017, and if you noticed, July 4th, clearly stands out as an outlier. 4th of july is the U.S. Independence Day and a national holiday when much less work was done, thus we are removing it from the dataset. Using the same approach, let’s also exclude several other “spiking” holidays, to get a smoother picture. Thus here are some of the dates we filtered out:

  • In 2017:
    • January 2 and 3, 2017 - since we don’t have the January 2 and 3, 2016 to compare to.
    • April 13 and 14 - the Easter Holy Thursday and Good Friday. April 17 also was cut - as it was the first monday after Easter, looks like lots of people decided to extend the long holiday.
    • May 1 - the May Day holiday observed in many countries, especially in Eastern Europe
    • May 29 - Memorial Day in the U.S.
  • In 2016:
    • December 26 - Monday after the Catholic Christmas
    • November 24 and 25 - Thanksgiving in the U.S. and the day after (a.k.a. “Black Friday”).
    • September 5 - Labor Day in the U.S.
    • May 2 - Orthodox Christian Easter.
    • May 9 - marking the end of WWII in Europe, observed as a national holiday in Russia, many Eastern European and ex-Soviet states.
    • March 25 & 28 - Catholic Christian Good Friday and the following Easter Monday.

There actually was another particular outlier on March 24th, 2016 that was neither a holiday nor a weekend. We attributed it to individuals taking leave to add a day to their 4 day Easter holiday. But since it wasn’t drastically lower, we kept this day in.


After the other outliers, dates above included, were filtered out, this is the picture we get.

Though much more smooth, the chart is still quite “spiked”. This time we will have to blame it all on Fridays. On average, Time Doctor users logged around 56 hours less every Friday compared to other weekdays.

As a matter of fact, if you are using Slack daily, chances are your activity in the app throughout a given work week is not consistent. For instance, the most Slack-heavy day of 2016 was Wednesday, followed by Tuesday, then Thursday, Monday and only then by Friday.

Despite those spikes, the overall trend is rather clear. On average, Time Doctor users were adding almost a thousand hours to their Slack use every month. By all measures this seems to indicate rather rapid and steady growth of time logged for Slack.

Or does it?

We’ll get back to this question in a bit. But first, let’s analyze the data we have for Skype.

Skype - the 800 pound Gorilla

Created back in 2003, Skype has been an ultimate game-changer for millions of its users, connecting people to their families and serving as one of the first tools for remote team management.

Even though it did not turn out to be the ultimate email killer, Skype became strongly associated with video and audio calls and instant chat messages for millions of users.

This Skype chart uses similar filtering logic as previous Slack charts. We removed weekends and major holidays from the picture. You can see that people all across the world were logging thousands of hours daily for Skype throughout all of 2016 and the first half of 2017.


As a matter of fact, if we plot Slack numbers alongside Skype, it is obvious how much more popular Skype still is.

On average, if Time Doctor users used both Skype and Slack on the same day, they used Skype 5.7 times more. And just as Slack’s usage grew among Time Doctor users in 2016, so too did Skype’s usage. But what about the intensity of that growth? And, most importantly, the growth of Time Doctor itself? As we have more and more people and teams taking control over their productivity and starting to use Time Doctor, we also see a growth in overall time logged by all our users. How do we exclude that factor from drawing any conclusions in a case of Slack vs Skype?

Comparing Apples and Oranges

The easiest way to compare these things is to calculate a “usage index” for the data sets.

A usage index is the relation between an average use of an app in a particular given day to the average use throughout the whole year. Here is the chart for the Slack usage index.


On average, a Time Doctor user would log close to 36 minutes (0.6 hours) of work on Slack a day. But that average is very normalized as usage varied a lot across the year. This is where the usage index is utilized. We take that average, 0.6 hours, and go through every day Slack was used, comparing it to the actual use.

Let’s use an example to illustrate our logic. Assume it’s some winter Monday evening and today you used Slack for 2 hours. This means your usage index is 3.3 (which is 2 divided by 0.6). Now, imagine yesterday you used it for just 14 minutes, which is 0.25 hours - now the use index is (0.25 ÷ 0.6) just 0.4. Think of it as the distance from 0 and 1, since 1 is what we get if time use equals the app average. For Slack, a use index of 1.2 would mean an app was used 20% more than average and 0.75 would mean it was used 25% less than average.

According to our usage data, Slack usage grew steadily month over month, having grown 2.6 times across 2016 alone. And as the chart shows - this trend is evermore present and stable.


Finally, let’s get to one of the most interesting charts.

Here we plot the Skype usage index alongside the Slack usage index - and can clearly see a trend. Not only is Slack growing much faster than Skype, Skype is almost stagnant if we compare it to the overall Time Doctor platform growth.


In this chart, we now added a third line showing the rate of growth for Time Doctor. As you can see, the Time Doctor growth rate and the Skype growth rate are almost identical. This indicates that Skype’s growth rate corresponds much more to just more people using Time Doctor. It does not necessarily indicate that Skype use is growing among Time Doctor users at all. And, of course, the upwards leaning Slack chart is a clear indicator of usage growth. No wonder why Slack is expanding geographically.


This is also very clearly seen if we zoom into current chart.

Before we continue, I just want to remind you that this data only pertains to Time Doctor users. If you were to look at how much each app was used globally, Skype is still way ahead of Slack. Actually, let’s bring that chart back.

Looking ahead

Let’s hypothesise for a bit. What would it take for Slack to overcome Skype, if we imagine for a moment that in the rest of 2017 and on trends have stayed and will stay the same?

To do this projection, let’s simplify things even more by comparing usage by month instead of by days.


Our two lines would look like this. On the left is the actual data for 2016 and a half of 2017, on the right, after the gap - a prognosis of where things will be if both Skype and Slack keep growing like they do.

Looking into the future, we see that Slack catches Skype around February, 2020.

But does this mean Skype is done and Slack will indeed soon become one chat app to rule them all?

Not necessarily.

This prognosis, though based on very real data, is still a rough estimate. It does not account for people switching from Skype to Slack, as Slack is constantly improving, adding video and voice calls to it’s platform to compete directly with Skype. They are even using a variety of landing pages to help generate awareness and grow their customer base. Nor does it account for folks going back to Skype who no longer need to pay for advanced features and simply want their chat history and calls for free. It does not include people switching to apps from other players, like Facebook with a version of its messenger for teams, Microsoft Teams and even other lesser known competitors like Flock.

This estimate does not account for ads and scandals, for laws and rule changes. And in the Internet of today all these things do inevitably factor into growth rate. It also does not address how people actually use those apps, it only analyzes how much time was used.

What if less times spent on Slack actually indicates that people find it easier to use than Skype? That maybe they use Slack for team communication and Skype to talk to their clients.

These are all questions for another analysis and another story.

But if we can say one thing for certain: it is that Slack is indeed on a roll and that if things stay the same this company is sure worth watching and even betting on (even Amazon was looking at acquiring them).


What Other Companies Say About Slack

  • “STATION F staff uses Slack for our internal communication as well as communication with the nearly 3000 resident startups, startup programs, VCs and public services on campus. Slack has a certain "street cred" in the startup community and our resident startups seem to love it.” - Joel Greiner, Communications, Social Media & Content at STATION F.
  • In spite of its tremendous growth, it is also worth noting that it is not for everyone. Tom Smith of says "I'm GenX and I personally find them to be a tremendous waste of time with users trying to outdo each other with memes. I find it distracting and interruptive to getting stuff done.”
  • “Slack’s growth is incredible. It’s really amazing how fast companies can grow in today’s connected world.” - Ty Magnin of

Thank you to those who helped us edit the article:

Chris Von Wilpert (, Kathryn Cave (, Matt Grech (, Rob Scott (, Simon Cocking (, Brandon Weaver (

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