It's more serious than that. Legally speaking, if the cops show up and tell you that a user is using your site for X, or Y, or Z, then from that point on... yo

It's more serious than that. Legally speaking, if the cops show up and tell you that a user is using your site for X, or Y, or Z, then from that point on... you know that your site can be used for X, or Y, or Z. How that knowledge impacts your liability in the event there are others using your service for similar ends, is not at all clear to me given the wording of the Act in question. It's kind of a "Notice, Knowledge, and Privilege" thing. And that gets really tricky in federal court.