Nick Denton shared at Valleywag the World Map of Social Networks
There are quite a few important countries and networks still missing, but ...
The map and country list by network at Valleywag is showing the strongholds of bebo, blogger (?), cyworld, facebook, friendster, hi5, livejournal, myspace, orkut, skyblog, studiVZ - business networks like Linked-In and XING are out-numbered (but still around).
Some addition / correction:
China = 51.com
Japan = Mixi
Russia = ckontakte.ru
via Jeremiah Owyang Social Network usage by Country
(cross posting on HEM on Media)