OkCupid, America’s largest free online dating site, has announced it has closed $6 million in Series A funding. The funding, provided by angel investors, will be used for continued development of new features for OkCupid’s free online dating services, and expansion of its service into key geographical markets worldwide.

OkCupid is the fastest growing free, online dating site in the United States with more than two million monthly unique users and 500,000 active users. Launched in 2004, OkCupid members take entertaining personality quizzes and answer compatibility questions to create their own customized matching algorithm. OkCupid’s proprietary matching system uses these answers and advanced mathematical algorithms to recommend compatible users. To date, more than 150 million matching questions have been answered by OkCupid users, and more than 100,000 new member accounts are created every month.

“Our growth validates our unique approach to online dating and the idea that online dating should replicate how people interact ‘offline’,� said OkCupid CEO and Co-founder Sam Yagan. “When we founded OkCupid, we knew that users would want a dating web site that provided a much better matching and community experience – without a subscription fee. This round of funding and the continued growth of our user base worldwide is proof that our approach to matching and community-building works.�

Yagan previously led two immensely popular consumer internet products, with varying levels of corporate success: SparkNotes, the online alternative to Cliffs Notes that was bought by Barnes & Noble in 2001, and eDonkey, the peer-to-peer file-sharing application that shut down under pressure from the RIAA.

Congrats to Sam Yagan and the team! This deal further erodes the notion that cost is the primary filter for many online daters interested in serious as opposed to casual dating.

This is great news for free dating sites, clearly putting additional pressure on pay sites, which will be hard-pressed to differentiate their offering from free sites once the marketing dollars and ensuing media coverage starts kicking in.

Read more at GigaOM.