I'm not sure that we can provably say that they LEAD to crimes. I think there's a strong case to be made that people interested in carrying out these crimes find like minded people online and engage in meme culture. You can probably make a good case that things like 8chan encourage copy-cat elements in these attacks.

I'm just not quite sure the case has been convincingly made that internet cess pools generate crime that would't otherwise happen. I could probably be convinced otherwise, but I'm not sure.

I actually hadn't thought of that side of it but you're right. This must be a good source to see what groups and where on the spectrum of "unhappy to planning sedition"...

Note that echochambers leading to enough hate is a problem for both "left" and "right" aligned people. The violent actions of some people claiming to be Antifa being a recent example.

Echochambers based on hatred (of Trump supporters, "SJW"s or black people) just leads to awfulness, what's needed is controlled exposure to average members of the hated group to cool them down.