Some social web sites are more popular than Facebook in some countries, here is a glimpse :

• South of America and especially BRAZIL : Sonico.com with 1.5 million of unique visitors
• China : Xiaonei.com with 17 millions of unique visitors
• Germany : Wer-kennt-wen.de with 7 millions if unique visitors
• Japan : Mixi.jp with 13 millions of unique visitors (To be registeryou need to be sponsored)
• Netherlands : Hyves.nl with 5 millions of unique visitors
• Russia : Vkontakte.ru with 16 millions if unique visitors) (They have invested $200 millions on Facebook)

And more….Take a look on this french easy article