4chan has had this problem for a while, but it can be hard to discern the real stuff from the edgy humor there. Sometimes I think the lines are so blurred that the difference is not all that important.
But many migrated to 8chan when Reddit and 4chan started applying some minimum standards, and 8chan is a whole other ballgame. Brenton Harrison Tarrant, the New Zealand mosque shooter, had ties to 8chan. Similarly, you have the old "classics" like Stormfront, Daily Stormer and Gates of Vienna which are definitely radicalizing.
Of the mainstream sites, Facebook definitely is the one with the worst problem. A lot of radical and extremist groups have their social media home there, from Islamist extremists to US right-wing militas. They are also notoriously slow to respond with moderation and often have divest far too little resources to handle it. You actually have reports of worrying degrees of PTSD among Facebook moderators; underpaid, overworked and with little to no network to ease the burdens of moderating extremist content.
In the US you had the Boogaloo shootings in may when two law enforcement officers were killed. One of the killers, Steven Carrillo, had a large following on social media which cheered his acts on as they were happening.
The rhetoric from extremist circles does creep into the mainstream. In Europe there has been awareness for quite some years regarding this phenomena. It is often done purposefully, through techniques like
ghost-skinning, extremists actively trying to gain entry into mainstream political parties, sub-cultures, law enforcement agencies or the military. The US, to my eyes, seems very naive about this development.