"You can't obviously sue a newspaper if it is reporting what someone else has said, and they have evidences he/she said it."
They need to be able to attribute the quote and if they do it becomes possible to sue the originator. The big difference between print media and the web is the speed with which the lie can br propagated by re-posting. That's something new. If anything it should put a greater responsibility on the likes of Facebook and, frankly, if they should only be operating at the scale at which they can contrive to meet it.