but in the end, figured
1) I didn't want to be embroiled in legal action (I'm assuming the person could sue me for slander)
2) I wouldn't check those sites when dating someone new, so it's probably not effective in terms of warning future partners
3) because there are no controls on who can post, that makes the information highly unreliable, which people who are accustomed to lying can use to their benefit (although one site I looked at required people to swear a legal affidavit)
4) one of the only things he cared about was maintaining was his reputation--if I took that away, he would have nothing to lose, and knowing he was capable of violence, that was another factor in not taking that action.
It's just a sad reality that unless the damage is able to be pursued legally (as in the case of the HIV transmission, or physical abuse/stalking/harassment) it's really not going to do anything except keep the person mired in the past.