There's a very simple line to draw. You police actions, not speech. Someone talks about building bombs? Fine. Talk never blew anything up. Someone builds bombs? Time to arrest them. It's not very complicated.

People are responsible for their own actions. "But he incited me to violence!" is not an excuse for violence. Maybe in the case of a superior giving a subordinate a direct order (in which case both are responsible), but "the man on Twitter told me to do it" should hold about as much sway as Son of Sam's "the neighbor's dog told me to do it".