This year, the world's largest social network will see a decline among teen users in the U.S., according to a forecast by EMarketer. It's the first time the research company has predicted a fall in Facebook usage for any age group.

Vanity Fair notes that for years Facebook has faced a lingering problem with one of its core constituencies: teenagers, the most fickle tech demographic, don't think it's cool.

Facebook, to its credit, saw the phenomenon coming: in a 2013 earnings call, then Chief Financial Officer David Ebersman acknowledged that teens were logging off the social network in growing numbers.

"We did see a decrease in daily users, specifically among younger teens," Ebersman said. And the problem hasn't gone away.

According to a 2014 survey conducted by Piper Jaffray, Facebook use among teenagers ages 13 to 19 dropped from 72 percent to 45 percent. In 2015, teens left Facebook because they found it "meaningless".