What happened

Shares of channel-based messaging platform Slack Technologies (WORK) fell sharply on Wednesday, declining more than 5% by the time the market closed. The stock's decline came as Slack filed an antitrust complaint against Microsoft (MSFT -0.32%) in Europe.

A chalkboard sketch showing a stock price falling

Image source: Getty Images.

So what

Slack's competition complaint argues that Microsoft is participating in an illegal anticompetitive practice of "abusing its market dominance to extinguish competition in breach of European Union competition law," the company said in a statement about the complaint. "Microsoft has illegally tied its Teams product into its market-dominant Office productivity suite, force installing it for millions, blocking its removal, and hiding the true cost to enterprise customers."

Some investors may be worried that this signals slowing growth for Slack recently as Microsoft aggressively markets its own workplace chat app, Teams.

Now what

Teams doesn't seem to have slowed Slack's growth in recent quarters. In the company's first quarter of fiscal 2021, for instance, revenue surged 50% year over year, and the tech company added 12,000 new paying customers and more than 90,000 organizations when including its new customers who are on a free plan.

For its second quarter, Slack said it expected revenue between $206 million and $209 million, representing 42% to 44% year-over-year growth.