The Business Technology Blog at the Wall Street Journal has a post on the topic of why online communiites fail. The post quotes a study done by Deloitte's Ed Moran which shows that there are three reasons why most online communities fail. But the fundamental reason is: Businesses are focusing on the value an online community can provide to themselves, not the community.
If you have an online community, or if you think your organization will have one in the next few years, I strongly recommmend you read not only the post, but the comments underneath. Doing so might save you a lot of grief.
Regards,
Glenn