Importantly, this study in unique because the participants were first tested in 2002, before Facebook was even founded. That enabled researchers to feel confident that people's experiences on the social network are a material issue in altering mental health. Brown University Professor of Epidemiology Stephen Buka says that the study "permits us to answer the chicken-and-egg problem: which comes first -- adverse experiences on Facebook or depression, low self-esteem and the like."
The team behind the study want to make the point that people shouldn't discredit online bullying because it's somehow just Facebook. Project lead Samantha Rosenthal says that people shouldn't "think of it somehow as less impactful because it's a virtual experienced as opposed to [one] in-person." Rosenthal goes on to say that because "people tend to feel more entitled to bully online than they do in person," there's a "higher risk" of genuine hurt being caused. That's why, if your Facebook experience isn't great, you should disable your account and walk away.
The wider point here is that online aggression is a seriously problematic element of the online experience. The Online Disinhibition Effect (or the Greater Internet Fuckwad Theory) is a documented phenomenon that allows people to be more obnoxious online than in person. Nations are just now beginning to work on criminal penalties for online abuse, and companies like Twitter and Facebook struggle to cope with the bearpit mentality. Until such time as a "solution" to this problem can be found, we're going to keep seeing studies like this reveal what we already know: online abuse can hurt.