Shelley evoked Field of Dreams last week, so I'll go back to Costner: part of the problem of the "If you build it..." approach is that it adheres to the old "stickiness" model of success which says, "To build a community, you go create a platform, and then you try to get people to come to it." It's a solution seeking a problem.
After all, if this is an era of spreadability, we can't expect to tame "community" to exist exclusively at a particular place or platform. Instead (to use two buzzwords I've associated myself with in one paragraph), we have to think about a transmedia approach to community-building in the digital space. How do people move from in-person collaboration, to virtual projects, to online discussion, etc.?
The scholarly digital network is multi-platform, and it waxes and wanes, from Twitter exchanges that arise leading up to a conference to a Facebook posting about a media artifact that draws intense discussion among a scholarly network. When the time calls for it, there might be a dedicated scholarly collaboration (as, for instance, happened in fan studies with the "Gender and Fan Studies" series of conversations a few years back). And you have sites like MediaCommons providing a steady stream of real-time, relevant content that can spread through those networks.
We can't build a system of digital community that requires intense participation from everyone all the time. Likewise, we don't want to close our conversation off to an "exclusive" membership without porous boundaries that don't allow others to discover us and naturally come to join the conversation. What's worse, as is often the bigger problem with creating a gated community, we run a far greater risk: cutting ourselves off from intersecting with new scholars/approaches/ideas by not listening to what's happening outside our walls.