Conversation Agent's Valeria Maltoni has a post on Measurement and ROI for Social Media that I missed last week but I noticed it today. 

"….Whether ROI equals return on investment, return on involvement or return on influence, knowing the "return" part matters a great deal. We talked about feedback before. Measurement is an important form of feedback."

In order to measure anything, it first needs to be defined - so I think, like the Branding example I gave earlier this weekend ( Thoughts on the Definition of Branding from Duct Tape Marketing - Part 1 and Thoughts on the Definition of Branding from Duct Tape Marketing - Part 2) - if you can define, first, what the ROI actions your looking for - you might be able to find a way to tie in Social Media activities to the increase/decrease of whatever you define as ROI.

The example I gave of measuring Branding can also be adapted to measuring Social Media ROI.