Rivals Microsoft and Slack are sparring yet again, this time over new growth figures that show Teams pulling well ahead of its top competitor.

Last week, Microsoft said Teams reached 20 million daily active users, a 50 percent spike in just four months. In the last month, Microsoft said Teams customers conducted 27 million voice or video meetings and performed more than 200 million open, edit, or download file actions.

At first blush it looked like bad news for Slack, which saw its stock drop 8 percent that day. But the news also provoked some online skepticism about whether Teams’ growth was real.

It wasn’t long before Slack jumped into the conversation. Just hours after Microsoft unveiled the new Teams numbers, Slack issued a statement, making the case that a big chunk of those 20 million people don’t actually use Teams very much. A spokesperson noted that Microsoft’s figures for usage in the last month equate to just over one voice/video call per month and approximately 11 monthly file actions.

“As we’ve said before, you can’t transform a workplace if people aren’t actually using your product,” Slack said in a statement. “Slack continues to see unmatched engagement on our platform with 5+ billion weekly actions, including 1+ billion mobile actions. Among our paid customers, users spend more than 9 hours per workday connected to our service, including spending about 90 minutes per workday actively using Slack.”

Last month, Slack said it had more than 12 million daily users, a 37 percent increase over the prior year.

Slack’s critique and the internet reaction to Microsoft’s latest Teams figures appeared to strike enough of a chord that the tech giant felt compelled to respond. Jared Spataro, corporate vice president of Microsoft 365, told CNBC this week that users are only counted when they perform an action such as making a call through Teams or starting a chat in the app.

Spataro addressed a recent criticism of Teams growth, that the program starts automatically for some customers, potentially raising usage numbers by including people who don’t actively use the product. Booting up Teams — whether by default or on purpose — is not enough to count someone as an active user, he said.

“We feel really great about the way we define daily active users,” Spataro told CNBC.

The rivalry between Slack and Microsoft is well-documented. It dates back to the launch of Teams in 2017, when Slack took the unusual step of placing a full-page ad in The New York Times both congratulating the tech giant and warning that “all this is harder than it looks.”

In recent weeks, the CEOs of both companies have bragged about their respective collaboration tools. And just last week, Slack seized on the popular “OK Boomer” meme to skewer its more established rival over a recent Teams advertisement.

Microsoft has acknowledged Slack as a major competitor in its annual report. And the Redmond, Wash., company even included the tool on a list of software that Microsoft employees are discouraged or prohibited from using, primarily for security reasons.

Slack acknowledged the uphill battle it faces against larger competitors, specifically Microsoft, as a significant risk to the company going forward.

survey of IT pros late last year found that Teams passed Slack in usage and trailed only Skype for Business, another Microsoft product. The company plans to replace Skype for Business as its primary meeting tool with Teams, which could speed up growth even further.