This is a typical situation. You start a community. You initiate discussions, you prompt people to participate in discussions. You invite members to join.
You reach 50 members, then 100. A majority of them continue to be active. You know every member.
Then you reach critical mass. That 100 members become 250, then 500. You gradually shift to macro-level activities. Activity keeps rising, but the number of newcomers which become regulars declines. The level of personal contact, which was so essential in keeping a member active, is unsustainable over the long term (or non-scalable).
You can't maintain active relationships with 500 people. The problem here is you failed to scale the community.
There are four elements of scaling;
1) Social scaling processes. These include actions performed by people. This means volunteers, insider groups, rituals, and habits.
2) Technological scaling processes. This includes notifications, automation, and physically handling the increased load without breaking.
3) Business/Organizational scaling processes. This includes how the organization interacts and integrated with the community. This covers feedback loops, investing, and growing the community team.
4) Personal scaling processes. This is how you personally handle growth. This includes how you allocate your time, acquire skills to grow the community, and take yourself out of as many processes as possible. How do you spend your time? Acquire skills to manage a growing team
You need to begin scaling in the establishment phase of the community lifecycle (not mitosis - that's too late).
Using our example above, you need to recruit volunteers very early, coach them to build relationships with newcomers and keep the newcomer rate high as you move on to new things. This is a social scaling process.
This isn't the only method, there are other potential social scaling processes too. You might also create a ritual for regular members to welcome newcomers every Friday, or a habit of responding to someones first post in the community with a unique welcome.
The goal is to maintain the same level of contact and quality whilst you move to manage processes and not individuals.