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.
