Teens are abandoning Facebook at a staggering rate, but they're still flocking to sister app Instagram, according to a new survey by Piper Jaffray.

Just over a third of teenagers report using the core Facebook platform at least once a month. That's down significantly from 52 percent of teens two years ago and from close to two-thirds of teens in Spring 2016.

What's more, Facebook is least popular among the youngest of users, hinting at tougher years ahead in capturing the key demographic.

But the company is clinging to the teen demographic with Facebook-owned Instagram, which edged out Snapchat as the most-used social platform by teenagers for the first time since Piper Jaffray started conducting the biannual survey.