Right now they are considered a platform, not a publisher. Platforms can't be sued for libel. They are acting as a publisher. Remember the Covington Boys, CNN (A publisher) etc. got the pants sued off of them for Libel. That is one rich kid. Anyway if it is determined they are a publisher they can be sued. So if you were to post that I'm an Orange NAZI on twitter. I could essentially sue you and twitter, if I prove that I am not an Orange Nazi I could win millions. Both you and twitter would pay. No one knows if you have enough money to pay out 8 figures but everyone and his lawyer knows twitter does.