SAN JOSE, Calif. — Mark Zuckerberg may have laughed off questions that Facebook is too powerful when he appeared in front of Congress last month. But for the companies in the online dating business, there was nothing amusing about the news that the social network was about to bring its heft into their arena.

At Facebook’s annual developer conference on Tuesday, Mr. Zuckerberg revealed that the world’s largest social network will soon offer an online dating service. More than 200 million people on Facebook identify themselves as single, he said, and the new service will let these people connect with each other from within the company’s primary app.

“This is for building real long-term relationships, not just hookups,” Mr. Zuckerberg said, in an apparent jab at dating apps like Tinder that have a reputation for stoking casual romantic behavior.

The reaction to Facebook’s dating app was immediate. Shares of Match Group, the company that runs Tinder and other popular dating apps such as OKCupid, plummeted more than 22 percent — the largest one-day drop in its history — after Mr. Zuckerberg’s announcement. Shares of Match Group’s owner, InterActiveCorp, or IAC, dropped by almost 18 percent.