You could try,
http://whitelabeldating.easydate.biz/ , I have never tried them so maybe you can let us know if they convert.
I have been told by several people in the dating game that Lesbian converts the best, but my experience so far has been much the same as yours. I think the problem may be if your sending porn surfers to your website, you really need to send women.
May I ask what your free signup ratio was to that 5,000?
Maybe someone here on GFY who has experience may be able to give some better advice on converting lesbian as I would also like to know.