You should always direct people to a landing page where they can choose from or be directed to multiple different instances.
https://kbin.pub/en or https://kbin.fediverse.observer/list
https://join-lemmy.org/ or https://lemmy.fediverse.observer/list
You can direct people away from specific instances if, for example, they have been de-federated.
Yes, the network effect is known to exist. The community is large, therefore it will continue to grow. On platforms/sites where the community is small, people will stay away, because there must be a reason not to join. Human nature is there opposite of load balancing.