I think that most Americans believe that Social Networking is a North American phenomenon. Nothing could be further from the truth. According to comScore the Asia/Pacific region is growing at nearly three times the rate of North America. Europe, a more comparable North American demographic, is growing at nearly twice the rate.
Social Networking sites and tools are a growing global phenomenon. While the current batch of "biggies" (FB, MySpace, Orkut, M5, etc) will certainly change with time, it's clear that the phenomenon will have a long (and wide) tail.