Do we want to listen? We can measure how much of the conversation we are listening to (and hopefully acting on if needed).
Do we want to talk with customers? We can measure how much they talk back.
Do we want to energize our customers? We can measure the viral spread of our brand message.
Do we want to support customers? We can measure the number of customer service issues answered through social media/number of calls averted.
Do we want to embrace customers? We can measure the product/service ideas we are getting from customers and how many we implement.
Suggestion: Only choose two or three metrics, at the most, to track for social media. Incorporate them into your marketing dashboard (you are looking at social media as another set of marketing tools, aren't you?) Follow the trends over time, rather than obsessing on daily changes.
There is also a great post today over at MarketingProfs on social media metrics.