Facebook is just about to reach 700 million users, according to Socialbakers, a blog that tracks Facebook statistics.
The countries that contributed most to the growth were emerging countries, especially Brazil, Indonesia and India. Brazil and India are interesting because they’re huge and have other social networks. If Facebook dominates there, it can grow that much faster and dominate that much more.
Note: During the Month of May,2011 Facebook Growth in Brazil is Estimated at Close to 2 Million New Users
See Also: Facebook Statistics By Country
See Also: Comparisons: Estimates for Number of Twitter Users Vary and Can Get Complicated (More Here via AllTwitter)
Bottom Line: Somewhere between 200 and 300 million users.