I use facebook to keep in the barest of contact with people I've known over the years (I prefer not to bother but I tell myself its good).
Yet, after replying to a friend's (actually, my ex's) wall post, I received a rather nasty comment from some random high school girl I'd never met (she seemed to be a friend of his daughter's). I never asked for this person to contact me, nor for her to be cruel, nor for her to indicate she knew things she never should have, things some very close companions don't know.
Why is this behavior acceptable, even among NTs?