Most people seem to find out about Mastodon from someone, and that someone suggests the instance they're on. That's why a lot of artists end up on Mastodon.art, for example. So far, the admin has a good reputation for being consistent about updates and keeping the community engaged.

People keep trying to find some technological heuristic to establish trust, like requiring legal names or some kind of scoring system. So far, it's even worse because people encode all their cognitive biases into it and none of the mental models or processes people develop for checking those biases.