One recent realization: I suspect many (read: more than just teen girls) see social media as a real-time status market. There is a constant stream of information, and the subtext of that is often status-oriented. Thus, for the status-hungry, keeping up with updates feels necessary even if it makes them feel terrible.
There are elements of addiction at work here, too, but if social media was a net negative it wouldn't be as sticky as it is now. People are obviously getting something they want from it.
Tobacco was a net negative and, without regulation, would have remained super sticky. Exercise is a net positive and, for many, is horribly unstick you. How sticky something is correlated better with how well it hits up those short term dopamine surges (or how well its absence causes short term dopamine deficits) than it does with the net value it provides a person.
There are elements of addiction at work here, too, but if social media was a net negative it wouldn't be as sticky as it is now. People are obviously getting something they want from it.