Since I'm currently exploring some of the same problems, I'd like to ask further..

How do you guard yourself against user-shared copyrighted content* ? Do you delete on request from the copyright holders? Do you weed out in the shared content once in a while? Do you have some kind of policy or agreement transfering the infrigement to the sharing user in such cases ?

*Say, a user has access to some content behind a paywall, but shares it un-blocked through your service.