LOS ANGELES (CBSLA) — Joining Facebook to keep an eye on your teen may no longer work now that the number of U.S. teenagers on the social media platform is shrinking rapidly, a new survey shows. Roughly 51 percent of teens ages 13-17 are on Facebook, according to a Pew Research Center report , which found 71 percent of teens were on the platform in a 2014-15 survey. “The social media environment among teens is quite different from what it was just three years ago,” said Pew researcher Monica Anderson, the lead author of the report. The number of Facebook users is far below the 85 percent of teens on YouTube, 72 percent on Instagram, and 69 percent on Snapchat. Among the report’s findings: teens from lower-income families are more likely than those from higher-income households to use Facebook. While Facebook is still the world’s biggest social network with an estimated two billion users, recent surveys have indicated it’s losing younger users to Snapchat and Instagram, which is also owned by Facebook. The platform could shed as many as two million U.S. users under 24 this year, but is expected to offset those losses with gains among users in older demographics, according to a report from research firm eMarketer.