I met someone on OKC, after being on there for months. Comparing our experiences on the site, she would get messages all the time, I had trouble getting anyone to reply to mine. Fairly typical male/female experience difference that's been talked about here, before. I finally deleted my OKC account in frustration, recently. Still good friends with the woman, although the dating part of our relationship ended...long story. So it wasn't a total waste of time for me, but one good result in 10 months is a lot of work for little return.
There's also PolyMatchMaker, for a poly-specific dating site, but the pool of active members seems small. http://www.polymatchmaker.com