I just wanted to make new female friends where I've moved, so joined both pof.com (PlentyOfFish) and OkCupid. I far prefer the latter, especially now that my interests have moved from purely platonic to wanting to meet some nice, conscious, clean/aware women who are interested in some kind of "open relationship". The difference in who's using which system may be localized, but here, at least, it seems like the POF users are more prudish, conservative/religious and are are mostly looking for serious LTR or marriage. OkCupid seems to have a much broader, more interesting mix, and the questionnaire system allows you to learn a lot more about potential partners. (I'm looking for something just serious enough that I'd rather avoid dating people who are clearly incompatible—on POF, it's much harder to tell.)