Falsely shouting, "FIRE!" in a crowded theater isn't free speech, and neither are Klan rallies led by chants of "Kill [your minority slur here]!" Both are threats to endanger public safety and the SCOTUS sanctions bans on them, though I'd generally expect the bans to be at the local rather than federal level. That's before we even get into "fighting words" as a valid legal cause for beating the snot out of someone.

There's a reason trolls are more common online than in the real world; you don't have to worry about people being trampled to death in a stampede from a website (though the site owner may have other worries during such an event... ) and the combination of distance, anonymity and common sense means retaliation is usually no worse than a few flames. Explicitly encouraging violence can be legally pursued, though generally I think it would have to be done under the local laws wherever the comment(s) originated; if someone in India goes on a killing spree because of what someone in California says on Stormfront, I can't imagine how the latter person could be prosecuted anywhere but his home state.

Of course, the easiest way to tell if anything said here violates RAFOs posted Rules is to click--oh, right....