Another comprehensive site is on Wikipedia:http://en.wikipedia.org/wiki/List_of_social_networking_websites
I understand that Nasza Klasa (our class) is popular in Poland, and I remember finding a LOT of people I recognized on that website, even some that are in the US but have family there. I remember that another SNS was popular in Costa Rica, but unfortunately I couldn't find it on the list or remember its name. At any rate, I think it's so fascinating that each region / country has its own interest in SNS. This is good because it connects people of those countries together, however, it's a shame in that it separates us globally (even though we're all connected) -- I guess this is why we should learn many languages!
I also found it interesting how Facebook, apparently, has some 640 million users, and the next biggest site has about 500 million in China. It's quite curious how we do not get any media coverage on these foreign SNS -- they will surely surpass Facebook as the world becomes more connected and technologically advanced.