The growth bodes well for the Microsoft's Office 365 subscription-based productivity bundle, a key part of today's Microsoft alongside the Azure public cloud.
Slack shares were down as much as 10% on Tuesday following Microsoft's announcement and closed down 8.4%. The shares are down more than 18% since the company's debut on the public market in June, and down almost 50% from their first-day peak.
"Slack continues to see unmatched engagement on our platform with 5+ billion weekly actions, including 1+ billion mobile actions," a Slack spokesperson told CNBC in an email on Tuesday.
Microsoft introduced Teams in 2016, two years after the launch of Slack. In July, Microsoft said Teams had more than 13 million daily active users, more than the 10 million Slack had in the three months that ended on Jan. 31.
With 20 million claimed users now, Teams has pulled even farther ahead. In October, Slack said its app had 12 million daily active users. Slack on Tuesday did not immediately respond to a request for comment.
Alcoa, L'Oreal and Telefonica are among the enterprises using Teams. In October, Microsoft CEO Satya Nadella said more than 350 organizations have at least 10,000 people using the app. Some people have switched to Teams from Slack, Microsoft Vice President Jared Spataro told CNBC on Monday.
Slack has pushed back on the way Microsoft counts active users, pointing to people's use of Teams for voice calls, among other things.
Starting a chat, placing a call, sharing a file, editing a document or participating in a meeting through Teams can get a person counted as a user, Spataro said. Having the app open when booting up a PC or closing the app doesn't count, he said.
"Our checks in the field indicate Slack will have significant difficulty further penetrating the enterprise given the significant competitive offering from Microsoft's [Teams] product that could slow growth going forward quicker than the Street is anticipating," Wedbush analysts Daniel Ives and Strecker Backe said in a Nov. 7 to clients. Ives and Backe initiated coverage of Slack with the equivalent of a sell rating.
Office 365 subscriptions include access to Teams, as well as applications like Word and Excel. Commercial subscriptions to Office 365 contribute to a metric called Commercial Cloud that investors have watched closely in recent years; Azure, the Dynamics 365 enterprise software and commercial LinkedIn products also contribute to the metric, which came in at $11.6 billion in the third quarter, counting for more more than one-third of Microsoft's total revenue.