I just wonder how much people really want to participate in 'brand' based communities versus issue based ones. Facebook works because it is generic enough to be what you want it to be as a user. With a brand based community there is always the idea that the brand sits in the background someplace - lurking. Also, while I agree with what you've said above, for most brands you have marketing managers commissioning these projects who have no idea what it takes to build and run a community and who therefore build no ongoing sustainability into the project - they run a community as if it is a campaign with a shelf life, whereas communities are meant to be ongoing and require ongoing commitment.