There are several sites that have been built specifically for poly personals and none of them seem to have gained any traction. Could be bad design, could be bad marketing-- I suspect it actually has to do with low numbers.
OKCupid does well because of the sheer number of members. As the polyfolk are treated the same as the monofolk and come up in searches, more people get made aware of polyamory and some are curious enough to explore it. That greatly increases the pool of potential partners well beyond having a site where all of the people who join are already poly.
So if you're going to build a dating site for polyfolk, it's likely best to include monofolk and keep them all jostling up against each other. You'll have to figure out how to stand out in the crowd of mainstream dating sites, though, and how to market effectively to build general membership.
When speaking of various forms of non-monogamy...it ain't poly if you're just fucking around.
While polyamory, open relationships, and swinging are all distinctly different approaches to non-monogamy, they are not mutually exlusive. Folks can, and some do, engage in more than one of them at a time--and it's all good.