>>chango+2g
Probably because it is the place most likely to be where you watch your friends live their apparently perfect lives without also being able to see their failings. Facebook is not the only place that provides that type of thing, but brand recognition in the headline draws attention.
Real life isn't without its own flaws, but the research shows an increase in mental health issues when social media is a factor. That increase is interesting, and worth studying, even if it is not the only place one can develop mental health issues.