I think you are right, because, at least from what I have seen mentioned in passing, OKC was founded by math geeks from Harvard so somebody more or less serious is in charge of the algorithm, data storage, uptime, etc., and, for another, it is very big and well funded (acquired by match.com which is probably the first and biggest paid subscription dating site, OkCupid | CrunchBase Profile).

As a result, it seems to make more sense to find a "niche" such as BDSM (or, anything on Earth) inside OKC than to look for stand-alone "niche" dating sites that do not have OKC's resources and client base and would most likely go out of business eventually. At least that with polyamory - somebody recommended some kind of a polyamory website on here many months ago; I visited it once - HORRIBLE web design, just horrible as if we were back in the 1990s, very few members, poor functionality, and the look and feel of a seedy place. So it will not attract users and might eventually go out of business. I subscribed to one of the national email list for polyamorous people two weeks ago and learned that they use OKC, and I have never seen any mention of the "niche" website. I have also never seen mentions of POF but do not know what it means. Maybe POF is more narrowly mainstream? I do not know - I have never been there.