In case we needed any more proof of just how bad Facebook's teen problem is, a new study suggests it could be even worse than we thought.

Just 51 percent of teens say they use Facebook, according to a study from the Pew Research Center.

That's a significant drop from 2015, the last year Pew published a similar study, when Facebook was the most popular social platform among U.S. teens. At that time, 71 percent of teens said they were Facebook users.

Now, YouTube, Instagram, and Snapchat are significantly more popular among younger users than Facebook. In 2015, Instagram was a distant second to Facebook, with 20 percent of teens, and Snapchat came in third with 11 percent. (YouTube wasn't part of the 2015 survey.)

While Facebook's problems nabbing teen users have been well documented for some time, the new study underscores just how quickly Facebook's losing ground in the 13-to-17-year-old demographic.

"Today, the majority use Instagram and Snapchat, which wasn't the case back then," says Monica Anderson, a research associate at Pew and lead author of the report.

There's some good news for Facebook, though. Instagram not only remains popular, it's slightly more popular than Snapchat.

Still, the report highlights what's become a worrying trend for Facebook: that no matter how much it copies Snapchat or builds teen-centric features for Facebook, most teens would rather spend their time online somewhere else.