So this argument you've made, you've just constructed a strawman.

> The users who quit were not randomly selected. Maybe they were receiving some kind of mental health treatment

You don't know that? You don't know anything about the selection process since facebook did not share their research. Your whole argument pins on the selection process you have no idea what happened. I'd find it very difficult to believe that researchers could not anticipate and control for situations like that. Researchers are after all, experts in research.

I don't know that, which is why I said "maybe."

Facebook does not typically do academic level research - they do quick studies to verify product direction.

From what I have seen, the actual academic studies on this are mixed. It is hard to say one way or the other, and it can affect different teens differently depending on how they use it.