Not much difference in distribution of users. There are a lot more instances to choose from. But a large number of users are on the main instance. For example in the case of mastodon, most of the user base is still on https://mastodon.online and mastodon.social.

https://matrix.org has the same problem. There are basically millions (I need to find the source for this number, I remember seeing it mentioned somewhere) of users on this instance and not even a number close to this on other servers.