In early November Microsoft announced a new product called Microsoft Teams. It’s a way for groups of people, typically colleagues inside a company, to communicate with each other over multiple, simultaneous conversations. It will be part of the software giant’s online Office 365 product, the “productivity” subscription program used by 85 million “knowledge workers” around the world. More than a billion additional customers use the offline version of Office.

A relatively small group of people—4 million, to be precise—will recognize something familiar about the new Microsoft offering. That’s because it’s more or less what a San Francisco startup called Slack does. Microsoft is adding a few bells and whistles, including easier-to-follow threaded conversations and video conferencing. Slack, which took the charmingly old-fashioned step of buying a newspaper ad to “welcome” Microsoft to its game, has said it will match those features. (Fortune, like many journalism organizations, uses Slack; after a year of steadily increasing usage, I’ve grown to like it.)

This isn’t the first time Microsoft has unveiled a “Slack killer.” In fact, it is becoming something of an annual event. What’s more, Slack is growing fast. It has 4 million users, up from 1.25 million a year ago. About 30% of those customers pay either $6.67 or $12.50 per month for the product, depending on which features they use. My back-of-the-envelope calculation of Slack’s annual revenue, assuming all customers pay the average of the two price points, is around $140 million. “You’re pretty close,” Slack founder and CEO Stewart Butterfield told me just before Thanksgiving.

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This David-versus-Goliath story could play out one of two ways. In the past, Microsoft users would ignore the new feature because it turns out what they really like about Office is email, Word, Excel, and PowerPoint. Slack would benefit from its focus. Then again, Microsoft has its act together in a way it hasn’t for years. And group conversations are the perfect complement to the other Office features. Slack could wither if users adopt Microsoft’s no-extra-cost feature.

It’s the kind of story only tech lovers and venture capitalists truly care about. Technology customers will benefit either way.