Jeremy Epstein contributed a must-read article about measuring social media ROI to this month's ASAE Marketing Insights e-mail newsletter. You have to be an ASAE member to read the article, so here's a highlight.
"...beware the traditional marketer in social media clothing. She cannot give you a quick and easy ROI response to your social media efforts using traditional direct marketing tools. What a marketer can do is to put on his quantitative hat and look for correlations between these metrics and the ultimate goal and then refine as necessary. By measuring the health of the customer-client online relationship and not focusing on moment-in-time transactions (e.g. traffic or hits), you can begin to develop an ROI measurement for your social media efforts."
And here's a link to Jeremy's excellent blog post entitled "Beware the Traditional Marketer in WOM/Social Media Clothing."