A list of social networks that get at least 1m visitors a day

Click to enlarge

Source:  Google Trends for Websites data, reported by Pingdom, 25th March 2011
Note - Google is pretty secretive about how they collate their Trends for Websites data (e.g. do they use Doubleclick stats?  Google Analytics stats?) so perhaps treat the figures with a bit of caution.  It's a handy list of social networks though!