The Social Metric Myth
This is a nice, balanced run down of the relationship between social metrics and social ROI. Essentially the author argues that social ROI can be measured, and for certain business objectives those tools already exist. However it is difficult to tie some of the more complex metrics, such as referrals generated or even value of a socially engaged customer.
One thing to keep in mind when talking about ROI is not to confuse it with (only) revenue. Sure, revenue maybe one of those objectives, but we should define ROI as impact on a present objective, not just dollars in the bank.