If you reported it to the FBI and nothing happened, there's likely a better thing going on. I just took a look at it. By the nature of the child-porn laws, writing provocative stories (text-only) about sexual perversions between adults and children or anything sexual including children, it is illegal and the FBI would jump on it.
So, either the content does not go over the line, OR its run by the FBI as it looks like a forum board. Thus, all content is brought to the site by signed-up members.
If the FBI online crimes and Child Sex Crimes unit is not at least culling the member list (that is, if they don't own the site), then they are darn fools...and they ain't darn fools when it comes to tracking pedos.
Think of the powerful list that place provides them. It also gives the feds probable-cause to further monitor the membership.
If there's no exploitative content, they (FBI) are likely using it.