[NoGravitas]

It's not very like that – subreddits are all on one server, but posts only appear on one subreddit, whereas Fediverse instances are separate servers, but posts propagate between different instances.

Mastodon instance administrators have the ability to block whole instances, and this is usually done because of bad/nonexistent moderation policies. There is some sharing of instance blocklists, more as a matter of convenience than of policy.