Activitypub and Nostr are the only actually decentralized networks.

* Bluesky is an elaborate deception
* Farcaster requires running terabytes of blockchain shit that saturate your upload and require retaining an entire copy of the network.

Nostr is a mess and loaded with spam but it basically works and is more or less decentralized.

ActivityPub has loads of problems but basically works. Most users on Fediverse are on a few big servers but this doesn't actually hurt you (yet) in the same way that not being on bsky.app does.