Liable for what? They already are liable if they enable things like CP if they don't proactively scan and remove it, and other crimes like distribution or sale

Liable for what? They already are liable if they enable things like CP if they don't proactively scan and remove it, and other crimes like distribution or sale of illegal substances is something they have to work closely with police on if they don't want to be held for aiding & abetting such crimes. They just aren't going to be the defendant if someone wants to bring a libel suit that originates from a false statement in a tweet, because Twitter isn't omnipotent and can't verify everything someone states to be true.