More than 20 million people now use Microsoft Teams daily -- up from 13 million in July. The product's impressive growth has many analysts and investors worried about the long-term prospects of rival collaboration vendor Slack.
Slack's stock was down more than 8% Tuesday in apparent reaction to Microsoft's announcement. The company's value has been steadily declining since June as more and more financial analysts have voiced concerns about the rise of Microsoft Teams.
Unlike Slack, Microsoft has a massive base of existing customers to target. More than 200 million people use Office 365 every month, and those customers usually have access to Microsoft Teams at no additional cost.
"Microsoft has the advantage of including Teams collaboration with a lot of their Office 365 packages," said Rob Arnold, analyst at Frost & Sullivan. "And that literally gets it in front of more users than Slack can ever hope to."
Microsoft also has a vast network of partners worldwide that provide services and support to businesses using its software. Slack launched its partner program last week, but so far has only recruited small and midsize firms.
Slack has attempted to undercut Microsoft's growing user count by focusing on user engagement. Among paid customers, Slack users spend nine hours connected to the app and 90 minutes actively using it each day, the company said.
"As we've said before, you can't transform a workplace if people aren't actually using your product," a Slack spokesperson said in a statement Tuesday.
There is, however, no evidence to suggest Teams users are less engaged with the app, as Microsoft has not released comparable statistics on the subject. Microsoft said Tuesday that users conducted 27 million voice and video calls in Teams last month, and interacted with documents stored in Teams 220 million times.
More than 12 million people used Slack daily in September. Use of the app has more than tripled over the past three years, making the vendor a leader in the market for team-based workplace communications software.
Slack often leads larger rivals Microsoft and Cisco in adding innovative features. For example, Slack developed a way to export emails to Slack in a few clicks earlier this year, while Microsoft won't launch a similar feature until early 2020.
But Slack has never made a profit, losing nearly $139 million on $400 million in revenue last year. Attaining profitability will require selling to more businesses with thousands, if not tens of thousands, of employees. Many of those companies already use Office 365.
In an interview last month with the Wall Street Journal, Slack CEO Stewart Butterfield said 70% of its 50 largest customers were using Office 365. He also pointed out that many of the top Google Search trends for Microsoft Teams were related to uninstalling the app.
Over the past year, Slack has been redesigning aspects of its user interface to be friendlier to the average office worker. The company launched the tool originally for software engineers, which led to quirks in the way users interact with bots and integrations.
This month, Slack is in the process of rolling out a new toolbar for writing messages that resembles what users are accustomed to when using apps like Microsoft Word. The toolbar lets users bold, underline and italicize text, and create numbered and bulleted lists. Previously, users had to do unintuitive things like put asterisks on either side of a word to make it bold.
But Microsoft has also been investing heavily in Teams, naming it the successor to Skype for Business. Just last week, Microsoft announced a partnership with Salesforce to integrate that vendor's online sales and service platforms with Teams. The move could further boost the adoption of Microsoft's product.