The ActivityPub protocol is a decentralized social networking protocol ...
ActivityPub specification abstract
The term, decentralized, is often used when discussing the virtues of the ActivityPub (AP) protocol. But what does that really mean? It seems that those involved in the discussions are typically thinking of decentralization in terms of computing resources. They also seem to believe that the big social sites are centralized in that sense, which is certainly not true.
The Fediverse (ActivityPub-based and others) and big social sites both utilize many computers, distributed globally. Both are "federated" in the sense that the individual computing nodes work together as an integrated system. It's not clear that ActivityPub has an advantage with this form of decentralization. It may actually have several disadvantages (less efficient communication, little or no failover if a user's instance is down, etc.).
In what sense does decentralization make ActivityPub different?
I think it's about decentralization of control rather than computing resources. ActivityPub instances are controlled locally and instance operators set their own policies and cooperate, or not, to create the AP Fediverse as we know it. This is in stark contrast to centralized control and policy-making of the large social sites. Our ability to effect change is very limited in that context. With the decentralization of control, we have options. We can find a like-minded community with local moderation policies that are consistent with our beliefs. We can even create our own instance and have complete local control over it.
Of course, decentralized control has limits. When groups of instances band together to block other instances (maybe ours), there is typically nothing we can do about it. Still, for me, decentralization of control is one of the best features of the Fediverse.