Some great stuff here from Wishpond as part of their ‘Social Success’ eBook. Worth a read.
Four ways to measure social success:
Fan count (but consider fan value (purchase value + referral value (vs. non fans)
Engagement rate (otherwise billed as conversation rate, share rate, applause rate, people talking about this)
Referral traffic to website (but consider also to store)
Sales (volume or value) (but consider broader economic value – money saved as well as money made)