Now imagine your entire friend network is nonstop angst. I'd go mad, too.
It could be a just-so story, but I think overuse of SNSs allows our social skills to atrophy. The brain repurposes structures that go underused, and the structures that maintain one's face-to-face social functioning are no different.
Maintaining relationships in realspace is different than maintaining them in the async-y & memetic environment of cyberspace. Of course there is granularity--one can have a healthy analog&digital relationship, but if a SNS connection is a part of an individual's requirement for a friendship, then there is probably some deficit the individual is unaware of.
I'd also argue that this is symptomatic of the growing pains heavy internet users experience in their adolescence-to-young-adult stage. The group self-selects for introverts who have a preexisting difficulty with social interaction. However, we all require some social interaction (analog or digital), to greater and lesser extents, and so it perpetuates.
Books and art end up being a better outlet, but in my experience that's a discovery which people can only make for themselves.
"The cross-sectional nature of the data precludes evaluation of temporality and causality of the observed relationship between use of SNSs and mental health problems."
So, when the press run with "Social media causes brain damage in children, think of the children!", remember what the paper actually says.
Personally, I think from anecdotal experience that heavy SN use fucks you up no end, but these findings don't prove this.