@sod BINGO! Wrote about this, too (I think on Mastodon haha). What most tech pundits and mainstream media who cite the discoverability issue of the fediverse as a whole, and Mastodon in particular, is that they don’t understand that it is EXACTLY the same as e-mail.
One thing that made Mastodon a bit complicated is the LOCAL and FEDERATED timelines. Posting locally on most Mastodon instances is also visible in the fediverse (unless you use HomeTown Mastodon-fork, which does not echo locally posted toots to the fediverse. Last I’ve checked this was not merged with the main Mastodon trunk). I think this is why Eugen (Mastodon author) decided that the Mastodon mobile apps hide these two, to simplify it for new users, and IMHO, to make it look like Twitter. :)
Mastodon’s list of instances adds to the confusion – one is dedicated for some topic/theme/what-not, but posting locally means posting in the fediverse anyway. NO point in looking for an instance that is dedicated to coffee, for example, since there is no local community to start with.