Microsoft said on Tuesday its workplace messaging app, Teams, has more than 20 million daily active users, up from 13 million in July.
The software maker offers the app as part of some Office 365 business packages, as well as a free version. Teams allows users to chat, share files, make calls and hold web video conferences.
Slack, whose customers include Electronic Arts and Nordstrom, reported more than 10 million daily active users in the second quarter ended July 31.
Slack's shares fell 8.4 percent following the news, to $21.18. They are down 45 percent from the close of their first day of trade in June.