You're right that Facebook was successfully abused by the Christchurch killer, but 8chan was being used by his community as intended, not abused.
To most people, that's why 8chan needed to be shut down, but Facebook only needs to be fixed. Why do you think that's irrelevant, and being small is the only crime?
I have searched and been unable to find anything suggesting this is true. Do you have a citation?
Another reason I don't think this is true is that I also can't find anything suggesting that the content the Christchurch killer posted is illegal (indeed, it has significant newsworthiness and academic value), and 8chan's policy was to allow anything not "illegal in the United States of America" [1] . If it didn't violate 8chan's policies, why did they take down the stream?
To most people, that's why 8chan needed to be shut down, but Facebook only needs to be fixed. Why do you think that's irrelevant, and being small is the only crime?