Some believe that communities form for a common purpose. Once the community has achieved that purpose, the community disbands.
I'm not sure this is true. There are strong communities around the world that have existed for generations without a clear purpose. Common interest trumps purpose.
Yet most communities (online or offline) do die, especially over the long-term.
They die for many reasons. For simplicity, let's skip aside the obvious blunders (bad moderation, spamming members, not initiating/facilitating activity etc…).
We're left with two major reasons why once successful communities die:
1) Lack of new members. Robert Putnam attributes the declines of most associations and community groups to the lack of fresh blood. The average age creeps up (average age is worth tracking). Once members begin to die out (or lose interest in the topic), the community hits a death spiral.
2) Lack of new topics/issues. Imagine members that have talked about everything possible within their common interest. Why would they continue participating? StackExchange once discussed the problem with a community freelancers. Every discussion was about how to find work. That gets tiresome fast.
It's important to ask two questions:
First, are we getting a good supply of new, active, members (at least enough to replace the members we're losing)?
Second, are the nature of discussions evolving?
Both require deliberate interventions driven by the community manager. The former requires to target new audience segments (and cater to them), encourage referral activities, and give more attention to the newcomers to the topic.
The latter requires documenting the most common topics, highlighting and facilitating discussions on new topics, understanding where the industry/sector as a whole is heading and ensuring the community stays relevant and fresh.
Communities don't naturally die, yet many do. If you want yours to last, you need to keep attracting fresh members and topics.
You can now buy my first book Buzzing Communities: How To Build Bigger, Better, And More Active Online Communities through the links below