Slack says that while its user numbers still lag Microsoft's, what really matters is that users love its app a lot

• On Thursday, Slack announced that it now has over 12 million daily active users.
• In comparison, its rival Microsoft Teams most recently reported over 13 million daily active users.
• Slack says users spend over nine hours a day connected to Slack and 90 minutes a day using the app.
• Slack also says most of its top customers are actually Microsoft Office 365, customers - which includes Teams - but they still actively decide to pay for Slack.
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Slack's daily active user (DAU) count has crossed 12 million - short of the 13 million most recently disclosed by its leading rival Microsoft Teams. However, Slack says that numbers don't tell the full story, and offers figures to suggest that users actually like using its app.

Slack, now valued at about $13 billion on the public market, announced this milestone on Thursday, ahead of its Spec user conference later this month in San Francisco. That 12 million DAU is up 37% from this time last year, Slack says.

Slack says that users spend over nine hours a day connected to the service, and 90 minutes a day actively using the app. It's Slack's position that figures like those matter more than any plain user number.

Read more: With 13 million daily active users, Microsoft says its Teams chat app is now growing faster than Slack

"DAUs (daily active users) get cited a lot, but what, really, is their significance?" Brian Elliott, vice president and general manager of platform at Slack, wrote in a blog post. "In our book, the 'U' is what matters: Use! Engagement is what makes Slack work - you can't transform a workplace if people aren't actually using the product."

In addition, Slack reports that every week, there are five billion "actions" on Slack, which accounts for sending messages, uploading files, searching, and other ways users interact with the app.

Office 365 customers are also Slack customers

Of note, much of Microsoft Teams' growth has been attributed to the fact that it's included as part of the Office 365 productivity suite for businesses. However, Slack now says that of its top 50 customers, at least 70% of them are Office 365 subscribers - implying that while they may get Teams included in their Office 365 subscription, they're still willing to shell out extra for Slack.

Slack also said that that large companies like Kaiser Permanente, Vanguard, and TD Ameritrade choose Slack because it supports an unlimited number of users and channels, whereas Microsoft Teams has a limit of 5,000 users and 200 channels per workspace.

"The way we work is fundamentally changing," Elliott wrote. "It's becoming faster, more adaptable, and more collaborative. This change is fueled by engagement."