Online, there are a few things needed to make a community succeed.  Great technology of course but I think the #1 thing is a lightning rod.
 
This was really driven home in this presentation looking at online communities.  There needs to be one person, or one personality who keeps the community growing.  It is not different than in the real world, where a rabbi or minister leads their congregation. 

One of the things I am exploring at Bear Ideas is how to help clients build and grow niche , local communities online.  The media giants have been swallowing up the very large online communities like myspace, friendster, flickr, youtube, facebook, etc.   And some newer ones focused on a particular area like technorati for blogs and yelp for reviews or 43places for travel.  All have large community components driving their content.