A few weeks ago I received an email from a former student. A man she knew from an online interest group had been arrested for sexual assault. Several members of the group ended their involvement, possibly over fear of who else in the group they did not know.
The thing to remember about online communities is that you often really don't know the other participants all that well. You come together because of one common interest, which is only one slice of any individual's life. Yes people sometimes go on to become good friends (or even spouses) having met online. For the most part, it's like any other community where you're only engaging with a subset of your self -- except in online communities you lack the visuals of body language to better assess the person.
It's easy to forget that individuals who commit rape (and other crimes) can also present as nice, cool, intelligent, charming, witty, fun people. And they may be all those things, in addition to being an assailant.