> Speech that leads to violence, how about starting with that?

That's already illegal in most (all?) of the US. The precedent for that is Brandenburg v. Ohio; the legal term is "imminent lawless action". (Regarding Facebook, my understanding is that they will happily remove such speech. That's what the report button is for.)

Of course that doesn't provide for censoring politically incorrect or socially unacceptable views so naturally people get bent out of shape and proceed to use terms such as "violence" and "harm" in creative new ways.