Especially if we're talking about, oh, say, bigoted views, I think it's pretty understandable that seeing that kind of thing, even if it's 'only online' will change someone's opinion. Yeah, one has the right to post what they want to Facebook, but if someone is posting things they know to be controversial, well, they can't get upset if it changes someone's opinion of them.
I've an old coworker who I always liked, and he posted something awhile back really nastily blaming sexual assault victims. Had it been a different way of phrasing it, I wouldn't have thought much of it, but now it's hard for me to see him in exactly the same way. It's not that I can't handle people with different views. I think most people have *some* view that they would see as extreme enough it would change the way they saw the person. It just might be a different line for different people.