1. It's free and I was penniless grad student.
2. Instead of the super-sanitized questions common on other sites, they use user-submitted questions, such as:
   • Have you ever played Dungeons and Dragons?
   • How man books do you own?
   • Do humans and modern primates (apes, chimps, etc) share a common evolutionary ancestor?
   • Would you be willing to go shopping for sex toys?
3. As a Computer Science grad student, I was impressed by their matching algorithm

In a nutshell, here's how their matching algorithm works. Each question is multiple choice. In addition to choosing an answer, you select one or more answers that your ideal partner would make. Also, you rate how important this question is to you. For all of the questions that you've both answered, OkCupid computes the match percentage according to the following formula:

On top of that, they apply confidence intervals so that people who have only answered a few questions cannot have artificially high scores. (Why can't Amazon do this for their product ratings? – a rant for another time.) My wife and I have a 94% match rating.

Anyway!

In the past year, OkCupid has started to generate a lot of buzz by mining their enormous database of quirky questions and writing about the results. For example, they've examined Why the Democrats have a hard time being as focused as the Republicans and Racism in dating.

This month, they decided to pummel their competition with an article titled “Why you should never pay for online dating”. They analyze publicly available data about eHarmony's and Match.com's to compute what fraction of their users are not paying users (96% and 93% at best, respectively). The key problem here is that non-paying users cannot write you back, so you have a 90+% failure rate on top of the already challenging task of finding a good date. Ouch.

I hope OkCupid makes a fortune.