There's just one other poly-friendly dating site I know of,
BeyondTwo. It seems to get good reviews, but I haven't tried it. I think your biggest disadvantage there would be that it's a rather new site, and won't have a very large number of members. Still one never knows what might work for you so that's why I mention it.
Fetlife reminds me of Facebook in some ways, while it also contains many forums some of which have some pretty in-depth discussion threads. It doesn't seem to have much in the way of filtering mechanisms (for deciding who you should message), but it seems like you can just about pick members at random and they'll probably accept a friendship request. Sometimes you can look at who's interested in a fetish that you're interested in, or you can look at who belongs to a group (forum) that you belong to (within Fetlife). [shrug] I don't think it's really designed to be a dating site, but it does seem to have an awful lot of members you can meet.
I have the same handle on Fetlife as here (kdt26417) and you can friend me there if you want; just let me know you're "poly6." Fetlife has a search window, and you can search for location (e.g. city/State) names as well as for usernames. I visit Fetlife regularly but I don't do much there; most of what I do is shadow my metamour who is quite a bit more active there than I am. I'm plain old vanilla and so I'm mainly there as a show of support for him (and for kinksters in general).
Beyond OKC, PMM, Fetlife, and BeyondTwo, my recommendations for you would venture into the realms of finding (a) local poly group/s in your area, joining, and attending its events. By making (platonic) friends who are poly, you may later on find yourself growing friendships into romances, who knows. If this approach interests you let me know and I'll provide some links and stuff you can use to aid in your search. But as far as poly-friendly dating sites go, I'm all used up.