As long as everybody are federated - but they are not. The bigger instances tend to be federated with everyone, but the smaller/more specialized ones might not yet have federated with each other. Of course you can still follow people from anywhere (as long as they're not actively _de_federated), but searching for hashtags might unfortunately be less powerful for discovery from a smaller server. If you're on a smaller instance not specific to your interests, you might do particularly well to start out finding accounts to follow through some third party list.

The greatest advantage of being on a specialized instance is, however, in the feeds: The "local" timeline shows you anything posted by anyone on your instance, while the "federated" timeline shows you anything posted by accounts someone on your server follows. On a large, general server both these feeds will be extremely crowded with all kinds of content. On a specialized instance, it's likely that both timelines will be somewhat interesting, and people might be reading it and discover your posts there as well. :)