When talking about centralization you can always of course zoom in the different levels. Decentralized systems are made up of systems that viewed individually can be said to be centralized.

BBSes viewed as an archipelago are decentralized.

Fidonet, a network to exchange mail between BBSes, was somewhat decentralized.

That’s a really good point that I hadn’t considered, thank you. When I spoke about bulletin board systems being decentralized, I was thinking more in terms of the interconnected Internet. Facebook is a centralized social network, it has tremendous reach. But a bulletin board system is one phone line (or five if you’re really lucky) and anyone can splin up a bulletin board system. The limiting factor is of course the phone lines, and if you don’t like one, you can go to another.