The McKinsey Quarterly:

"Many myths have been laid to rest since the Internet bubble burst, but a cherished if previously untested principle—the idea that an on-line community can increase the "stickiness" of a World Wide Web site and the value of users to it—is proving true. A McKinsey–Jupiter Media Metrix study of visits to World Wide Web sites by 40,000 consumers shows that chat rooms, bulletin boards, product review pages, and similar community features create substantial value for both retail and content sites."