Most people access the Fediverse through one of the large instances: lemmy.world, kbin, or beehaw. New or small instances of Lemmy have no content by default, and can most easily get content by linking to larger Lemmy instances. This is done manually one "Community" at a time (I spent 15 minutes doing this yesterday). Meanwhile, on larger instances, content naturally aggregates as a result of the sheer number of users. Because people generally want a user experience similar to Reddit, I think it's inevitable that most user activity will be concentrated in one or two instances. It is probable that these instances follow in the footsteps of Reddit- the cycle repeats.
I actually think the Fediverse is in the beginning the process of fragmenting into siloed smaller, centralized instances. Beehaw, which is on the list of top instances, just blacklisted everyone from lemmy.world. Each of the three largest instances now are working to be a standalone replacement for Reddit and are in direct competition with each other. It is possible that this fragmentation and instability? of Lemmy instances will kill the viability of Federated Reddit altogether, but hopefully not.
These are my main takeaways from my three days on the Fediverse. I will stick around to see if the Fediverse can sustain itself after the end of the Reddit blackouts.
There's already an entire Star Trek instance, startrek.website. Niche instances may end up being a way some communities get around the issue of big servers defederating from each other over their different administration policies. With that topic having its own instance, Trek fans on Beehaw and Lemmy.world can unite in nerdy fandom across the divide. I'm not quite sure how the defederation works through a third party instance though, if posts/comments from Lemmy.world users to startrek.website will still show up when viewed through beehaw's display of the communities there or if they will be filtered out of the feed locally.
I'm scrolling through old posts and figured I'd give an answer to this in case someone else comes across it and is wondering. Regardless of if the instance is a third party instance that's federated with two instances that are defederated (ex: beehaw and lemmy.world), neither can see the other user's posts or comments regardless of where they are.
I was scrolling through [email protected] earlier and there is a popular post that I can see from another account, but I can not see it on my beehaw account, because the poster is a lemmy.world user.