In this instance I don't think calls to violence are really the problem at hand. Specifically, what is the difference between getting on a box on a street corner and calling for the death of so and so and saying the same thing on the internet? The difference is the speed at which the message propagates and the breadth and targeting of the audience that hears it. The technology and its unregulated use are the problem not primarily the speech it facilitates. Facebook and Twitter in how they are designed are the problem. They are the equivalent of giving everyone an information machine gun with infinite advertising backed ammo and then asking everyone to abide by the honor system. We don't do this in the physical space and we shouldn't in the digital space. We are currently in the process of learning all of the old lessons of society building the hard way when it comes to the internet. Thats because Twitter and Facebook didn't intend to build societies they intended to make money. They did both but only really cared about the latter.