In South East Asia, I would say that Friendster is a leader, even if MySpace and Facebook are obviously very present. Friendster has opened offices in various southeastern countries (Philippines, Indonesia, Singapore). Hearing that it is still kinda strong in Japan (ok, nothing compared to Mixi), even if it's fading against newly-comer Facebook.

As for China, Facebook has had some good successes. There is a government-approved SNS (the name eludes me right now) that is growing rapidly. Same as Facebook: it started as a student thing.

In Europe, Bebo is popular amongst english-speaking people, i.e. mainly UKI, even if it offers translations. I've read somewhere that people do not engage as much on it, they tend to quickly check it and then go.
Netlog has been growing in other european markets (well, bar my french friends, they're crazy on Skyrock), as its localization services are well done and engage teenagers not yet able to speak the international language. I think it's one to closely watch.

The other one that it's often forgotten is hi5, a network I'm not even on I think (what a shame). It works well in the south of Europe, Africa and Latin America (bar maybe in Orkut-land, Brazil). In Asia, Thailand comes to mind, but I don't know about other countries over here.

I don't see the Obama administration actually considering all these options. Far too time and money-consuming for external PR. They will certainly go the Facebook road. Now, if they want to make friends in some of the parts of the world where some anti-US feelings were high at times, they should think about the chatter on some other SNS.