The overwhelming number of communities you hear about have hundreds of thousands or members. They grew overnight in a huge explosion. Facebook has 500m members in four years.
This is a huge distortion of reality. These are the outliers. They get the most attention, but they don’t reflect the truth.
The overwhelming number of successful communities grew slowly. They began with a few members. Then they attracted a few more members. The creator invited select people and groups at a time. They invited people with something to contribute. They invite people who they specifically wanted to join an online community. Others just happened to chance on by.
If you’re banking on 10,000 active members in your first year, you’re probably going to be disappointed. You’re going to aim for big press coverage and major victories. You’re going to miss your expectations, become depressed (instead of focusing on existing members) and quit. It happens a lot. Don’t bank on rapid growth, ever.
It’s much better to predict 2000 members in your first year if you’re personally inviting 5 – 10 people a day to join. That’s a defensible figure. Once you have a realistic number (and treat extras as a bonus) you can decide how much content, time and resources you need to spend to get there.
So set figures if you need to, but justify them with defensible actions. Ignore the outliers.