Teen mental health is plummeting and social media is a major contributing cause [pdf]
I wish these studies would stop working with the base assumption of “we know social media is bad, we just have to prove it”. I’m not saying this conclusion is wrong, but when you start with that biased view, you end up doing bad science. It’s not enough to prove correlation, you need to prove causation. 
Kids who are depressed have more time on their hands, fewer friends, fewer activities, it’s almost a given that their social media consumption must be higher even if there is no effect. 
But the studies never seem to spend much time considering this because they aren’t trying to figure out if social media is bad for mental health, they already decided this is the case and they are just gathering ammonitikn for their predefined conclusion.