If it were a country, the site would be the fifth most populous, larger than Brazil and Japan. In a blog post Wednesday about the milestone, Facebook co-founder and CEO Mark Zuckerberg wrote: "Both U.S. President Barack Obama and French President Nicholas Sarkozy have used Facebook as (a) way to organize their supporters. From the protests against the Colombian FARC, a 40-year-old terrorist organization, to fighting oppressive, fringe groups in India, people use Facebook as a platform to build connections and organize action."

To mark the occasion, Facebook has begun a campaign selling a collection of virtual gifts for the benefit of 16 charitable and advocacy organizations including UNICEF and the UN Refugee Agency's "Gimme Shelter" campaign. It also set up a site for people to share stories about how Facebook helped them effect change or connect with a distant relative.

Facebook's biggest challenge remains monetizing its ever-expanding audience, 70% of which is outside the U.S. Market research firm eMarketer projected that Facebook would break $200 million in revenue last year -- at $210 million -- but that figure came after twice cutting its original estimate of $305 million due to the recession.

The company last week said it was pleased with its financial performance. According to an internal memo cited by D: All Things Digital, Facebook expects to increase revenue 70% in 2009 and be cash flow-positive in 2010.

Adding a half a million new users each month becomes costly, however. Facebook is reportedly trying to raise another $100 million in debt financing to lease more servers to keep pace with its torrid growth. Facebook has also hinted that an IPO is in its future.

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