Tell it to Napster.
File sharing: Perfectly legal, almost all networks everywhere share files in some form or another. However, this particular service was used primarily to distribute copyrighted MP3's.
Napster totally in the right and the end-user is to fault would you not agree? Funny how Napster got totally dragged down by the RIAA for providing the means of illegal activity.
Your site also provides the means of allowing the end-user to go places they should not, so would you not say you're in a similar boat?
Logically you'dd think only the end-user is only to blame, but sadly reality tells us this is not the case. You're lucky that you had the foresight to keep accurate logging (or at least so you tell us), because (as someone previously mentioned) if they followed the trail back and it stops at you, who do you think gets incriminated?
Sure, you may be 'innocent' and not accessing that content, only providing the means.... but it still puts you at risk, and ultimately you're held responsible for your actions.