René Algesheimer (Universitat Zurich) and Prof. Paul Dholokia (Rice University) have studied the behaviour of eBay customers who belong to the online customer community, comparing them with other eBay customers who don't.
The full report is 46 pages long but Francois Gossieaux was kind enough to do a brief summary of the findings in the blog The Future of Communities.
The results are extraordinary and just confirm how important it is for companies to understand, encourage and support communities.