I came across this graph on the preferred use of social networking sites from Le Monde, courtesy of O'Reilly Radar. You probably need to click on the image to see it in full size to be able to read it, but it shows which networking sites are the biggest in different countries.
MySpace is the biggest in the US, Venezuela and Italy, while Hi5 is the most popular in Central America, Portugal and Mongolia. Like most Norwegians I prefer Facebook and so do the Canadians and the Egyptians. Orkut is most popular in India and Brazil, while Bebo is most popular in Ireland and New Zealand
In most cases there is no pattern, like language, connecting the countries with similar preferences. In stead I suspect it is all about being the first to reach critical mass. When you consider which social network to join you are obviously most likely to join the ones where you friends already are, creating a snowball effect that benefit first movers in the area.
I believe there is a lesson in this for us as well; if we want to succeed in building something similar inside a company, you need to get the snowball rolling...