Your suggestion is tantamount to demanding that gun manufacturers be held accountable for gun deaths. Facebook is merely a platform placed - for good or for ill - at the disposal of users, and should be under no obligation to "patrol" websites and "police" posters and/or scrutinize their postings for possible "political incorrectness."
The people who make a decision to be evil - you know: the actual perpetrators - should be held accountable for their actions, and no one else.
It’s not like that at all.
When you walk out the door with a gun the manufacturer looses all control for what happens.
The content on Facebook resides on their premises and servers. They own it.
That is the key difference.