Watching the #forkiverse come online is a good example of how to populate the Fediverse. One reason why https://a2mi.social has lasted for all these years is because we started with a core group of people who all wanted to talk to each other. It sidesteps the empty room problem.

I still expect a lot of people to drop off after the newness wears out but I also suspect a core group will stick around and slowly grow.