That's not true, and it's notable that you had to create a new account just to make this FUD claim. Those murderers may all have used Twitter and Facebook to try to disseminate their idea vomit, but they were radicalized and incited by 4chan and/or 8chan. Also, Twitter and Facebook make the effort to remove those posts once they are made aware of them. 4chan and 8chan revel in keeping up those promote, and even promote them to show just how "free speech" they are.
It's a bunch of anonymous people posting crap, often for shock value. Sure, there's incitement on there, but nobody takes anybody serious.
More than a dozen mass murderers in the past few years took 4chan and 8chan seriously enough to murder hundreds of people. Mass murder might be an abstract concept to basement dwellers, but I know people who were shot at in the Santa Barbara, Vegas, and Thousand Oaks shooting, and it sure as fuck isn't an abstract concept to me or the thousands of other people who knew the victims of those shootings.