The buck stops where it cannot be passed along any further.
Every word. Every video. Every picture. They must all be the responsibility of someone or something when spewed across the internet.
If I hire a car and then use it to rob a bank, then the hire firm is not liable. But if the hire firm failed to carry out rudimentary checks, making it impossible for the police to trace me, then the hire firm is complicit to the crime.
If Facebook is effectively allowing anonymous posts, that cannot be traced back to an individual or organisation, then it should be Facebook up in the dock.