A new study has confirmed what we've long expected:Facebook is no longer the most popular social media site among teens ages 13 to 17. The Pew Research Center revealed on Thursday that only 51% of US teens use Facebook. That's a 20% drop since 2015, the last time the firm surveyed teens' social media habits. Now, YouTube is the most popular platform among teens, about 85% say they use it. Not surprisingly, teens are also active on Instagram (72%) and Snapchat (69%). Meanwhile, Twitter followed at 32%, and Tumblr's popularity (14%) remained the same since the 2015 survey.