I don't know about Digg but StumbleUpon might be useful for your purposes. According to their website "StumbleUpon is the 2nd largest source of social media referrals, sending more traffic than the remaining social media landscape combined." They state that this info comes from StatCounter.com's Global Statistics. I'm sure Digg and any of the others can probably be useful as well but you definitely want to hit the larger social media players as well.
