This info graphic pretty much sums up the size and the shift in power of social networks in 2010. You can see that sites like Habbo, that are not as popular in the UK, are surprisingly huge worldwide (it's larger than Twitter at 178m users). This map may change rapidly though as new networks such as Apple's Ping are emerging all the time.
Many thanks to Paul Bradshaw for sharing this and bringing it to my attention.
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Hat tip for the thumbnail image > Some rights reserved by stevecadman