One Tuesday night in October 2018, John Catsimatidis, the billionaire owner of the Gristedes grocery store chain, was having dinner at Cipriani, an upscale Italian restaurant in Manhattan’s SoHo neighborhood, when his daughter, Andrea, walked in. She was on a date with a man Mr. Catsimatidis didn’t recognize. After the couple sat down at another table, Mr. Catsimatidis asked a waiter to go over and take a photo.

Mr. Catsimatidis then uploaded the picture to a facial recognition app, Clearview AI, on his phone. The start-up behind the app has a database of billions of photos, scraped from sites such as Facebook, Twitter and LinkedIn. Within seconds, Mr. Catsimatidis was viewing a collection of photos of the mystery man, along with the web addresses where they appeared: His daughter’s date was a venture capitalist from San Francisco.

“I wanted to make sure he wasn’t a charlatan,” said Mr. Catsimatidis, who then texted the man’s bio to his daughter.