I think the problem is that people have a tough time understanding communication without the nuances. Facial expressions, sound, context etc get lost on Facebook easily. Add to this that those who post often don't know how to frame what they want to say very well and it's easy to see how things go sideways.
However, I suspect those that fly off the handle easily on Facebook, would probably do the same in real life/face-to-face in similar situations with only social morays holding them somewhat in check.
I'm reminded of people who say, "I am just telling the truth" or "I just keep it real" who don't grasp there is a difference between telling the truth and being hurtful.
Either way, I really enjoy Facebook, and I like to think I use to MY full advantage without being hurtful etc etc.