I've seen some people compare Mastdon to IRC, I guess in the sense that IRC networks are (sort of) federated and anyone can run one. But IRC networks are closed and don't interoperate with other networks, so it's not a good comparison.
@DialMforMara Reddit is also centralized, though (a federated version of Reddit would be Usenet). The point I was trying to make is whether the protocol of a system inherently lets you connect to other systems with minimum trust required.