I have always thought of 4chan, at least their 'random' board, as akin to a sort of performance art. It reminds me of the experimental film scene in the 70s and 80s in NYC. The theaters were filled with prostitutes, pimps, thugs, drug addicts, etc. The experimental films which were shown were not anything that could be shown to mainstream audiences, things like Andy Warhol's 'films' that were downright pornographic, or just a camera pointed at the Statue of Liberty for 24 hours, just experiments with the medium. Also shown were works by the likes of Alejandro Jodorowsky and others who went on to create other bizarre cult classics. While the things being done in those theaters had no real place in the mainstream, many of the people in the audience at that time as children or young adults went on to become the most popular directors and influential filmmakers of the 90s through to today. There is no question that they were grimy, deviant, subversive places where crimes of various sorts took place. But they were also important artistic and cultural crucibles, and the experiments that took place there sometimes happened upon techniques or ideas that later enriched our world. It's not always pretty to see how the sausage is made.
The Factory milieu produced at least one would-be murderer, and probably worse. Not that I agree with the original comparison. Culturally, the Warhol crowd were a stop between the somber, black-clad Existentialists of the 50s and the somber, black-clad ‘goths’ of the 80s.
Facebook does get the blame. Many people on HN at least do blame Facebook for spreading hateful propaganda and fake news and for fueling acts of genocide. Twitter gets blamed as well.
But the chans are unique in that their culture (ironically or not) actively encourages and demonstrates the beliefs that lead to such violence. The shooter in this case mentioned having been inspired by /pol/ and mentioned the New Zealand shooter, who is being treated as a hero and a martyr by /pol/.
Facebook and other social media get blamed for passively encouraging violence and hate, while the chans get blamed for actively encouraging it as a cynical and ironic piss-take on society.
According to FB, nobody flagged the video during the livestream, and it's not clear how FB could automatically detect the nature of the content as it's being streamed. You might as well blame Subaru for manufacturing the vehicle used by the attacker.