This map shows the pattern of Facebook friendship links across places around the world, with lots of white where there are very dense links across nearby places. The map was created by a Facebook intern, and I learned about it (where else?) on Facebook (HT Mari Kuraishi).
One interesting pattern is a kind of Facebook Curtain somewhat related to the old Iron Curtain. The whole area including the former Soviet Union and China, along with other adjacent autocracies like Burma and North Korea, is pretty much a Facebook void (see zoomed map below). This reflects some combination of language barriers, preference for other social networks in Russia and China, and some (rather unclear) role for Internet censorship by the authorities, which either prevents or lowers the payoff to participating in Facebook.