Just four months after Microsoft Teams reached 13 million daily users, the company is reporting that number has reached 20 million.

Microsoft Teams is a unified communication platform that competes with Slack, Google Hangouts and Cisco Webex Teams. Not surprisingly, since it’s part of Office 365, Teams integrates with Microsoft’s other applications.

Slack, in contrast, reported last month that it had 12 million daily users. While that is substantially less than Microsoft’s 20 million, Slack claims that its users are far more engaged than those of Teams and other platforms. Even so, that hasn’t prevented Slack’s stock from taking a hit in the wake of Microsoft’s announcement.

CNBC reports that Wedbush analysts Daniel Ives and Strecker Backe told clients:

“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.”

According to CNBC, “Ives and Backe initiated coverage of Slack with the equivalent of a sell rating.”

Slack has estimated the workplace collaboration market to be worth some $28 billion, leaving plenty of room for multiple companies. However, to survive the competitive edge larger players bring, Slack will need to continue to evolve build on its strengths.