The total feasible audience size (TFAS) changes how you approach your community.
If the TFAS is small (say, <1k members), then your plan for growth is more relaxed.
You grow a little slower. You spend longer building relationships with each individual prospect before inviting them to join the community. Your time allocation shifts towards relationship development.
You probably won't grow via promotion or search channels. You will reach out to all 500 members personally. You spend the time building the connections first. If they're high value (like most most professional B2B communities), you might want to invite them all to an event (and foot the bill).
It's important to showcase that this community is attracting exactly the right target audience (and not just everyone).
Retention is paramount. If you lose 10 people that's 2% of your total possible audience gone forever. If you lose 100, that's 20%.
You can worry less about spam, removing bad posts, and generally moderating the community. There won't be as many discussions – one person can probably handle it. You need to design and plan your community accordingly.
If your TFAS is larger (usually >10k members), then it's important to establish a foothold, to highlight what makes this community unique, to showcase momentum, plan for spammers and handling 10k+ comments per week.
In these communities you initially grow via direct invites, but you soon switch to word-of-mouth and outbound promotion.
Most of the people you're trying to reach know if they're likely to be 1 in 500 or 1 in 100,000. You need to adjust your efforts accordingly.
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