Google [online dating]. That's why I say they're very good at SEO.

As to why they have that ranking? Well, they do linkbait so well that almost isn't fair to call it linkbait any more. They also do other things. One particular tactic which used to be quite popular in dating is viral quizzes.

http://www.okcupid.com/the-death-test

They have toned down the aggressiveness of that tactic in recent years, probably after a different site got torched for it. (Plus you don't have to be quite so aggressive after you've already won -- then you can use self-reinforcing authority and translate your commanding market position into, e.g., a competitive moat like their blog posts.)

They also execute very, very well. For example, it's easy to say "Viral apps win" and it is hard to be Zynga. Similarly, it is easy to say "Get more links and you'll win" and it's hard to top OKCupid's execution on that -- most individual tactics I've seen them try have execution in the top 1%. They're #1 with a bullet at linkbait. Those quizzes are better than 99% of quizzes. Their use of the product to support the SEO is better than 99% of startups. (Humming a few bars: OKCupid has rabid fans amongst folks whose dating behavior results in banhammers at other dating sites, and they leverage that to the hilt.)