| Having participated, observed and created many online communities, I feel I have some insights into their way of evolving.
Not having a crystal ball, nor a sociology degree, and not being Hari Seldon, I can only offer this as an unofficial aphorism.
I am fond of the Epic In Three Acts structure; so sue me.
- Prologue: Deciding to Supporting Interaction
The creators of the online service decide that the end-users are able to explore, contribute or manage the space together. That is, end-users will be able to see and affect each other in some way. This sets the life-cycle in motion; and sets it apart from any canned service that offers no meaningful interaction between visitors.
- Plot Point I: "Mostly" Feature Complete Service Goes Public
At a predetermined time, or at the moment of a mention on a popular publication, the service hits the big-time. At this point, the service will either erase any persistent records for the vanguard, or will choose to let them keep any accrued points, toys, contributions or other cachet.
The former approach will cause much whining from the vanguard now, but the latter approach will cause much whining from the soldiers who join after this point later.
- Act II: Vanguard and Soldiers Mingle
Throughout the arc of the life of the service, those of the vanguard will appoint themselves as keepers of the faith, and will try their hardest to control the techniques and actions of the soldiers who joined later. The soldiers will return fire, trying everything that the service technically supports, even if it's against the spirit of the concept. Cliques will form. EVERY new feature added to the service will be seen as a negative influence by most of the vanguard. One interesting point: I can't think of any major community that shut down over a disagreement amongst vanguard and soldiers. Such clashes actually benefit the community in some way; perhaps its a reason for the passers-by to return or get involved.
- Plot Point II: Shadow of a Competing Service
Any good idea will spawn competition. Either someone else was already developing something similar, the creators of this service are reinventing their wheel, or a clique of the vanguard or soldiers think they can do it better. News of an alternative spreads through the community like wildfire. Regardless of whether there really is a good alternative, the weeds will burn, the oaks will resolve to stay. Soldiers and vanguard alike must weigh their current investment in the service.
- Act III: Showdown
The devout oaks do their best to weather the storm, but it's a hot week when the competition opens up. Many will try the new service. Some try for a while to be devoted to both-- they have investments in friendships and efforts, after all-- but essentially it is a moment that decides if there is a critical mass or not.
- Epilogue? Schroedinger's Cat
If there is a critical mass of devoted visitors to the original service, the service will thrive. The landscape will be forever changed. It is almost assuredly nothing like the creators envisioned. These devoted people become the new vanguard; differences between "pre-release" vanguard and "post-release" soldier will vanish... until new soldiers come in and restart the cycle. Without the critical mass of people, all doing what they want to do, any community will die off.
Now, think about Everything 2. Or EverQuest. Or your favorite IRC channel. Can you identify the key moments in the life-cycle of your favorite online communities?
Cross Reference: cancer tumor growth patterns, viral infections, Wal*Mart vs Township demographics data, Environmental Impact of Logging and Mining Operations, Microsoft vs Dept. of Justice. | 
