What would we want beyond defamation law? We have defamation law, and I can sue and have something taken down. Do we want rules beyond that when it comes to things other people have posted? Defamation law does just that: it strikes the balance between freedom of expression and protecting reputation.
I wonder, are we talking about different rules beyond defamation law when we talk about what other people post? I think I'm clear on the first two paragraphs.