When Instagram announced a new feature last summer called Stories, it looked awfully familiar. Users could share photos and videos with their friends, which would appear for 24 hours before disappearing. There were filters, and stickers, and a pen tool to draw pictures and text over each image. There was even a progress bar at the top showing how much of each story you had watched.

It was, in other words, a near exact clone of Snapchat.

It wasn’t the first time that Facebook, which bought Instagram in 2012, had copied Snapchat, and it wouldn’t be the last. In March, after folding Stories into Messenger and WhatsApp, Facebook C.E.O. Mark Zuckerberg introduced the disappearing photo and video feature to the Facebook mobile app itself, giving more than 1 billion people access to his Snapchat-killer.

This aggressive strategy—Zuckerberg’s revenge, perhaps after Snap C.E.O. Evan Spiegel turned down his $3 billion acquisition offer in 2013—has apparently paid off. On Thursday, Facebook announced that Instagram Stories now has 200 million daily active users, surpassing the 161 million Snap reported in its last filing. That’s a 33 percent jump since January, when Instagram hit 150 million daily users, showing just how quickly Facebook can increase engagement by leveraging its massive user base against competitors.

Even before Facebook released its latest numbers for Stories, investors and analysts were worried about Snap, which made its public market debut in March. Earlier this year, TechCrunch reported that Instagram’s own version of Stories was having a major impact on how people use Snap. Social-media managers said in January that they’d seen Snapchat views decline from 15 to 40 percent since Instagram Stories launched in August. Worries that Facebook could continue to take a bite out of Snapchat weighed on the stock in the weeks after its I.P.O. “Snap Inc. is becoming a public company just as its user growth and monetization growth rates are beginning to meaningfully slow,” wrote Anthony DiClemente at Nomura Instinet, which gave Snap a sell rating.

Snap’s stock slumped further Thursday on news that Instagram had surpassed Snapchat’s daily user numbers. Shares fell to $19.88 in the afternoon, closing in on a historic low, before recovering slightly, to about $20.20.

As if to twist the knife, Facebook also announced a bunch of new features for Stories on Thursday, all of which mimic Snapchat: a sticker tool called Pinning, which lets users create augmented-reality “3-D stickers”; and “Geostickers” for a handful of cities including London and Chicago, which can be applied over pictures, like a Snapchat geofilter. Snap’s first-ever quarterly conference call has been scheduled for May 10.

Senior Citizens Struggle to Figure Out Snapchat