http://fairuse.stanford.edu/Copyrigh...pter6/6-b.html
Any site that accepts user submitted content (even forums) are liable, but are also protected by the law. The person submitting the copyrighted work is the one that is breaking copyright law, or in the case of minors (child porn), the same is true. However that does not mean you can't be held liable if you do not take down suspected piracy works, or if a provider submits a DMCA you should probably listen to them.