Via Andrew Sullivan, Vincos Blog has a world map of social networking sites:
With areas of dominance in North America, Latin America, Western Europe, Africa, and Southeast Asia, Facebook is the clear imperial hegemon in the world of social networking (and all that is despite its having one of the dumbest names in the business). But, according to Vincos, it is not actually the largest social networking site in the world; that superlative belongs to QQ, which dominates China with "300 million active accounts."
Good ol' Friendster, meanwhile, has been pushed back - a bit oddly - to a final redoubt in the Philippines. Hi5, which I had never heard of, has probably the weirdest distribution of strength: it's tops in Mexico and Central America, Ecuador and Peru, Portugal, Cameroon, Romania, Thailand, and Mongolia - and nowhere else. I defy you to find the family resemblance that ties that group of countries together.
There are several country or language-specific networks that are king in just one nation: Hyves in the Netherlands, CyWorld in South Korea, iWiW in Hungary, and Mixi in Japan, among others. A few networks are popular across a cultural region, like Maktoob in the Middle East, V Kontakte in the core areas of the former Soviet Union, and Odnoklassniki in the more peripheral areas of same. MySpace, meanwhile, has fallen from its perch everywhere but Guam.