Facebook implemented automation to ban any reposts of the NZ footage. Facebook routinely bans extremists groups.

8ch actively caters to white supremacists, by letting /pol/ be the second largest board there and not be actively moderated to combat hate speech. 8ch administrators did nothing to prevent hate speech from becoming a large chunk of their discourse. As such, people were looking towards 8ch's providers to do something about this, as it has been the case on the Internet since forever (ISP not reacting to abuse requests concerning their customers? Okay, we have to contact their upstreams then).

Perhaps only if commenters glorified it? I'm sure you can find people to do that on FB or anywhere else, and it may take time to moderate such commentary. But no one would deplatform FB or Twitter -- they're too big. This approach means we have a powerful dynamic in place to limit the reach of any social media that isn't already "too big to deplatform".