SAN FRANCISCO— Twitter ’s gloomy quarterly report last week unsettled investors. They might have anticipated trouble more than a year ago had they noticed one key indicator.

Until late 2014, Twitter was regularly ordering ping-pong tables from Billiard Wholesale, a store in San Jose, Calif. Then, suddenly, it wasn’t.

The store’s owner, Simon Ng, figured it either ran out of space “or they’re having company problems.”

Twitter Inc.’s slowing user growth has been unsettling analysts, and the company’s revenue growth was unexpectedly weak in last week’s report. Asked why Twitter stopped buying tables, spokesman Jim Prosser says: “I guess we bought really sturdy ones.” Twitter spokeswoman Natalie Miyake says: “Honestly, we’re more of a Pop-A-Shot company now,” referring to an indoor basketball game.

Is the tech bubble popping? Ping pong offers an answer, and the tables are turning.