A new article in the Harvard Business Review discusses a controlled experiment recently conducted at eBay Germany. 140,000 active customers were selected, with about half being invited to participate in eBay message boards. About 15,000 of the invitees became active participants or lurkers.

Those that participated in community became much more active participants in eBay, representing about 55% more activity and several million dollars of new profit. This well-designed experiment directly supports the value proposition of consumer communities.