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To add to Kevin’s reponse, I think Barnes & Noble has done a terrific job of integrating the community with their website so that any visitor to bn.com is aware of them.  My experience is that lack of promotion is the number one reason that communities fail — customers just aren’t aware of the social features you are offering them.   Therefore it’s rarely a problem of convincing people to join — if you have enough users and if you do a good job of creating awareness, they will figure it out!