It seems reasonable to define the "main Fediverse" as the set of servers that most other servers federate with. There are distributed blacklists of servers like

It seems reasonable to define the "main Fediverse" as the set of servers that most other servers federate with. There are distributed blacklists of servers like https://joinfediverse.wiki/FediBlock https://joinfediverse.wiki/FediBlock, and I would not consider the servers listed there to be on the "main Fediverse" since they cannot communicate with many of the larger Fediverse servers.