I'm only speculating, but if "everyone's on instagram", maybe people don't want to be left out?

They may know that it's bad for you, just as I expect most smokers to know smoking is bad for you.

But the "badness" isn't direct, it grows, and they could think "maybe I'll be able to control it / I can quit whenever I want", whereas if you don't follow current trends (or whatever it is people follow on instagram), you're left out immediately.

Of course, by the time you realize you can't quit, it's already too late.