Ok, consider matrix which is federated e2e chat. There are a few instances but the “dominant one” is matrix.org, the public test instance setup by the matrix devs. You probably do not want to use this one unless you absolutely have to. The reason being is that the instance is so large that chats take a while to load and sync and there can be some downtime as the servers are overloaded lots.
You can instead run your own little instance with no sign ups, just you, and still chat to everyone on matrix.org as well as the other federated matrix instances. Bonus, when matrix.org goes down, you can still chat with users on your instance and other federated matrix instances instead of waiting for matrix.org, your chosen, “dominant” instance to come back online.
This is a mental trap folks get into. Centralized services suck and are antithetical to the web’s design.
Think of these federated instances like email and ask the same question: “which will be the dominant email service? Gmail? Fastmail? Aol? Protonmail?” The answer is you choose the one you want for the reasons you want, and don’t sweat it because it will likely communicate across the rest of the internet (unless blocked by spam filters).
does the instances values somewhat align with mine? I mean to say if you consider joining threads as your fediverse instance, you mind find less content or a worse interaction with content in general? If you join a right wing server as a leftist, you might find the only content you can access is content you don’t want to see, as other instances have blocked that instance…
think they will be around for a while? Think again! All of lemmy instances are run by volunteers. If you don’t mind instance hopping when one goes down, just pick one. Guessing which instance will have the resources to continue in years time isn’t something you’re going to get a good feel for years to come. Lemmy content isn’t going to easily monetized meaning likely most instances will need to rely on donations in some form to pay the datacenters who literally keep the lights on.