Generally speaking, social media sites are not liable for the content posted by their users. I think you would have to be able to respond to copyright takedown requests, but that's about it. But I'm not a lawyer.
Just thinking about what you are trying to do, I suspect you'll have much more challenges before you even have to worry about legal ones. Creating such a web site for one will be a major challenge. Then, getting people to join will be another. People don't care to join a community site that doesn't have people, so you have the chicken-and-egg problem.