Sure, people always had stupid, dangerous ideas and seething resentments. But in olden times, they would think them up, tell a friend or two, accept the eye rolls they got in return, shake it off then return to their business of collecting the carts in the parking lot. Then the internet came along. Now some maniac can post his bizarre concept and get thousands of comments saying "That's a good idea, you should do that." The internet is like a dating app for terrorists. You're not getting the best. Also, you can't outsource outrage. But fanatics haven't realize this; their ability to judge others is compromised. If some internet guy says he'll help carry out a plan, the fanatic is so happy to have that support he won't realize that the internet guy is a buffoon.
On the other hand, having all these doofuses in one place certainly makes it easier for law enforcement to find and catch them. And as long as they don't have money, they will be punished. So, the only thing we really need to be careful of are rich terrorists...oh, wait...