www.engagementdb.com provides case studies on companies' use of social tools as well as a database used to analyze the effectiveness of different efforts.  The site is produced by Charlene Li of the Altimeter Group and Wet Paint.  Their new report shows a strong positive effect from social channels.  Or if you prefer the highlights, here's a write-up at OnlineMediaDaily.com.  One finding that stands out:

As the number of channels increase, overall engagement increases at a faster rate.

Social is one of those tools where more really is better.