Just to respond to "(Mastodon) has the most users (so it's the most social)."
I know this takes a bit of time to get used to, but federated platforms aren't in competition with each other.
When one federated platform grows, that also increases the audience for the other platforms that federate with it. As long as cross-platform federation is maintained, then success of one platform helps the others.
Because of this co-operative nature, you really don't need to check which Fedi platform has the most users when deciding which one to use.
To take it to an extreme, if a developer created a Fedi platform for themselves that literally no one else used, but it was still compatible with Mastodon etc... well then, they can be just as social as anyone on Mastodon, because they can still follow people, people can follow them etc.
The Fediverse co-operates because because it helps us all, but sincere co-operation is a concept that doesn't exist in Twitter, Facebook etc.