http://www.ignitesocialmedia.com/resources/whitepaper-measuring-roi/ Slides from a webinar with Tom Webster (http://www.twitter.com/webby2001) and Jim Tobin (http://www.twitter.com/jtobin) discussing 6 models anyone can use to measure the return on investment from social media marketing.
1) The Amplification Model;
2) Value of Social Media Traffic vs. Display Advertising Traffic
3) Quality of Visitors from Social Media Traffic vs. Display Advertising
4) Calculating Revenue from Facebook Fans
5) Calculating Revenue from Social Media Marketing
6) Social Media Promotions Sales ROI