I don't think that's really true - most of my friends who have used it were looking for potentially long-term relationships.  Most of them have also complained about how hard it is to find long-term partners on OKCupid.  So I think it's more likely that OKCupid is <i>used</i> to find long-term relationships, but it's <i>successful</i> at casual flings.  (Which makes perfect sense, since long term relationships take a long time and a lot of effort, so naturally you'll see fewer of them.)