I bet the outcome of the study changes over time. Sure, Facebook can be a mean place, like the school playground, but you tune it out (both mentally and with software) over time.

If you look at how national populations react to internet ads over time, behavior changes, and the same goes with Facebook. In the case of ads, people mostly just become desensitized, and I bet the same goes for the bile on Facebook. Not to mention how the website changes over time, in response to measured user behavior.