Beyond the short-term interest/need that compels a member to join, long-term participation is driven by four needs.
1) Power (efficacy)
2) Fame/recognition (appreciation)
3) Friends (affiliation)
Your members will participate because they want to have an impact upon their surroundings (even online), want people to say nice things about them and feel appreciated, make friends or feel a sense of achievement.
If the majority of your active members have been in the community for less than three months, you have a motivation problem. Your members don’t feel they can satisfy their power, fame or achievements needs within your community.
This doesn’t mean you need to give recognition, power etc…to these members. You just need them to feel they could quench their motivation thirst in these areas.
The difference is important. You can’t quench everyone’s motivation thirst in a scalable way. Nor should you try.
This isn’t an excuse to be lazy about giving members recognition, power or helping the community achieve things together. It’s the opposite.
It’s a compelling reason to be highly active in giving recognition (interviewing members, mentioning members in news posts, celebrating their milestones etc…), power (control over areas of the site, responsibility for certain topics, initiations to the volunteer group) and that sense of achievement (milestones, activities external to the community, collaboration activities), amongst a diverse group of members.
The more members see others being recognized, given power and achieving things within the community, the more they will feel they can get the same. The more they are likely to participate to achieve what these members have.
It’s the motivation thirst that will drive long-term participation.
Don’t try to be fair in dolling out recognition, power and achievements amongst all your members, you don’t need a rigid criteria for giving recognition, you just need to be highly active in doing it