Will Pate and colleagues have put together a good summary, researched online, of metrics that could be developed into something. I think the term ROI is a stretch for some of these items, and have said so before, but the concept of new metrics to respond to this space is highly valid.

Some examples that I liked:

– Community users remain customers 50% longer than non-community users. (AT&T, 2002)
– Community users spend 54% more than non-community users (EBay, 2006)
– 56% percent of online community members log in once a day or more (Annenberg, 2007)
– Customers report good experiences in forums more than twice as often as they do via calls or mail. (Jupiter, 2006)

For the full list, go to Wills post here.