Microsoft CEO Satya Nadella at the Fast Company Innovation Festival in New York on November 7, 2019.

Microsoft said Tuesday its Teams communication app now has over 20 million daily active users, up almost 54% from Microsoft's prior announcement about usage and still ahead of Slack.

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.