What makes a community thrive and what dooms it to failure?

In my career to help in designing communities and sorting out what things people would like.

I spend a great deal of time on these social networking sites, researching them and considering what makes them successful or "ghost ships".

I think there are two keys to successful community.

1. One is what I call the Beach Ball theory.

In a crowded concert or anywhere there is a mass of people who are likely to get bored and disengage.
The challenge of building thriving online communities and social networks is to find the "beach ball" that will engage potential users.

Giving them something to do, to rally around while they bide time waiting to connect with others. Points of conversation work effectively in the social network sites I audited. For example starting conversational strings for users to provide short, quippy answers. Allowing users to monitor activity on their profile.

more later....

2. The Other is the ultimate goal of true human interaction.
Most online communities at one point or another lcoalize to the point where the users wish to "meet-up" face to face in smaller, more specialized groups.