Excellent Question! One that I am currently working towards in my efforts to build a social community around crowdfunding.

A great book I read is by Seth Godin called Tribes. I found this slide share presentation that gives a good summary on the book and the elements described therein should help you on that: http://www.slideshare.net/hhoefele/notes-on-seth-godins-book-tribes-presentation

From my own experience:

1. Needs a structure for the community to communicate to collaborate

2. Needs a common purpose where the members can both further a cause or help each other if that is the cause

3.. Must reward members for positive behaviors and deter members for negative behaviors.

For my own site, I am using http://www.socialengine.com which is a white label social network that is working really well for us. To review what we have built you can check out incubator.realitycrowdtv.com and see the engagement and community we are building.