That seems pretty straightforward to me - there are a huge number of teenagers who use Facebook, some of whom say that it improved their mental health and a smaller group who say it made it worse. The media reporting only talked about the teens who said Facebook was harming their mental health rather than the broader picture. Which, if accurate, would mean that closing down Facebook would actually hurt teens' mental health overall but anyone getting their news from the mainstream media would falsely think it'd help. This kind of deception using facts that're technically true is really common in the mainstream media, and Facebook in particular has pissed off journalists by hitting their industry hard both in the pocketbook and in terms of the amount of power they hold in society.