Dating sites have probably the best retention among those sites that rebill. heck, one of my best friends is a former Laker girl and she is on Match.com and EHarmony so that should tell you something.
and just to spam a little i've done really well with Adult Friend Finder and
LoverCash if you want a adult-dating site.
i'm in the process of building a directory of online dating sites which i think will do very well....