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.