Here's a great article to read on Ten Measures of Social Media ROI For Your Brand. The main takeaway is the misuse of social media by many to just amplify / count follows and not use social tools as a way to further integrate themselves into their community and improve their business process.

The comments below are thought-provoking, read them, they bring up new points and enhance the discussion on what exactly IS your ROI / measurement!  It's a discussion I often have with clients, and all these points made within the article and comments are worthwhile to consider.