I know in my friends nephews case.. he's rather isolated. His family is rather large and people do visit very often, but he either isn't allowed outside (other than to get the mail) or doesn't want to go outside. I've been around him a lot and he friend requested me on Facebook. He's very considerate and thoughtful, he's never been rude or said anything out of pocket to me. Nor have I watched him "rage" but, I believe it all to be there and easy to trigger.
Most of his facebook stuff is about pokemon, football and sex. He doesn't seem to grasp the social appropriation lines, but that can be said about MOST social media users

. I only notice it with him because of the things that have been said to me by his extended family and his own mother.
When his dad suddenly passed on this summer, it was very worrisome how he would be told at the hospital. They didn't want him to hit a nurse or create a larger tragedy out of what was taking place already. I know they took him to a psych ward floor in a visiting area and told him and everyone was proactive that staff knew not to touch him etc etc. My entire point though, is that rage plays a huge factor with him and I know of a few that this seems to be what often gets them into trouble. Because of that, facebook and other sites like it are their outlets. They can safely communicate and interact without "harming others".
I want to touch on the sexual abuse thing for a second. I truly agree with those with mental disabilities are far more likely to be abused, not tell and to act out aggressively. However, in my friends - nephews case, he has never been abused but he's never had sex and is very sexually aggressive. I have been told this is rather common, and in SE classes the teachers have to finagle a way that compulsive masturbation is handled. The reason we believe the nephew could sexually harm a child is because of the things he posts online, some stuff that was found on a family computer (there was another male in the household at the time - the blame is passed back n' forth) and he hits on close relatives in a very aggressive manner. he doesn't see why he can't. It's very scary.