Simple fact... the FBI isn't monitoring what's going on here. They have bigger fish to fry. HOWEVER, if someone reports a user, supplies physical evidence and makes a big enough stink, the FBI *might* look into it depending on how serious it is.
Some examples...
Someone on here writes a story about how they've been having sex with their daughter since she was 10yrs old. Someone prints out a copy, sends that along with a link to the original content to the FBI. Hard to accept but this would probably be ignored. Too hard to prove, not enough evidence for a quick conviction and could all be here-say.
Someone on here writes about molesting his daughter since she was 10 and posts pictures of it. Same scenario as above. However if multiple people report it, then it might be taken more seriously.
I don't think people really understand how limited on resources the FBI old task force is.