Everyone around me is always searching for the ROI in social media and asking "How much are we making", " how much is that worth" and "what's the bottom line" while searching for answers online and in white papers. I've been searching for these answers as well, while developing my own perspective on social media ROI at the same time and haven't seen many come up with a good answer just yet.
My take on it is this - while there needs to be focus on the data and the metrics, companies like Facebook and Twitter need to be helping with that, or bigger companies can create some crazy amazing product that tracks everything for us... that's going to take time.
Until they figure that all out, everyone else needs to figure it out on their own, dig a little deeper into their own social media fans to discover who's a fan, who's a spammer, and who are you most loyal activities (who are most likely also spending the MOST money by far) and decide how you want to keep track of what this means to you.
Social prospecting is one of the best ways to do this. Keep track of your most loyal followers, those who scream your name from the rooftops and share every moment of their lives using your product or visiting your store with their followers, and go from there.