Well it still is their responsibility. It's like Dr Frankenstein letting the monster lose on the village and then going "hey guys well I sure don't know how to figure this stuff out".
If the government, the military, academia, the police or anyone else would let their inventions lose on the world in the manner the private sector does and leaves it to us to figure out how to clean up the mess we'd call them insane and demand them to be shut down within a week.
I'm no fan of Facebook, but in their defense, this is sort of the point.