Two Visions of Decentralized Social
The rise of interest in decentralized social networks has brought two architectures to the forefront: Mastodon (and the broader ActivityPub Fediverse) and Bluesky (built on the AT Protocol). Both aim to give users more control and reduce dependence on centralized platforms, but they make very different technical bets about how to achieve that goal.
Understanding these differences matters for users choosing a platform, developers building on top of these networks, and anyone thinking seriously about the future of social infrastructure.
The Fundamental Model: Federation vs. Separation of Layers
Mastodon / ActivityPub uses a federated model: each server (instance) is an independent node that stores its own data and communicates with other servers using the ActivityPub protocol. Your data lives on the server you chose to join. Servers interoperate through a push-based message passing system.
Bluesky / AT Protocol uses a layered architecture that separates data storage from application logic:
- Personal Data Servers (PDS): Store user data in signed, portable repositories called repositories — think of them as personal append-only logs.
- Relays: Crawl all PDSes and aggregate the full network firehose of events.
- App Views: Consume the firehose and build specific views (like a social feed) on top of it.
Identity and Portability
This is where the architectures diverge most significantly in practical terms:
| Aspect | Mastodon (ActivityPub) | Bluesky (AT Protocol) |
|---|---|---|
| User identifier | @user@server.tld (server-dependent) | DID (Decentralized Identifier) + handle |
| Moving servers | Partial: can migrate followers, history is lost | Full: move your repo to a new PDS, DID unchanged |
| Identity root | Server controls account identity | Cryptographic DID (did:plc or did:web) |
| Domain as handle | Not standard | Native support (your domain is your handle) |
Bluesky's use of DIDs means your identity is cryptographically anchored and not controlled by any single server. In Mastodon, if your instance shuts down and you haven't migrated, your posts are gone.
Algorithmic Choice and Feed Architecture
In Mastodon, your feed is chronological by default. There's no algorithmic feed from the platform itself (though some third-party apps experiment with this). The home timeline shows posts from people you follow; the local and federated timelines show posts from your instance and its federated network, respectively.
Bluesky introduces a concept of Feed Generators — open-source, third-party algorithms that anyone can write and publish. Users can subscribe to custom feeds built by the community. The architecture cleanly separates the data layer from the algorithmic presentation layer, enabling an open marketplace of feed algorithms without requiring changes to the underlying protocol.
Moderation Approaches
Mastodon's moderation is instance-level. Server admins set their own rules and can defederate (block) entire servers they find harmful. This creates a patchwork of moderation policies — useful for niche communities, but inconsistent across the network.
Bluesky introduces Labelers: services that apply labels to content or accounts. Users can subscribe to different labelers (think of them as moderation plugins), and the Bluesky app can filter or warn based on those labels. This separates the moderation policy layer from the protocol itself.
Openness and Ecosystem Maturity
- ActivityPub/Mastodon has a much larger ecosystem: Pixelfed (photos), PeerTube (video), Lemmy (link aggregation), Misskey, Pleroma, and dozens more all federate with each other.
- AT Protocol is newer and still primarily centered on Bluesky itself, though third-party clients and tools are emerging rapidly.
- ActivityPub is a W3C standard; AT Protocol is an open specification developed primarily by Bluesky PBC.
Which Is "Better"?
There's no universal answer — it depends on your priorities:
- If you value an established, diverse, multi-software ecosystem with strong community governance: ActivityPub / Mastodon.
- If you prioritize cryptographic account portability and an algorithmically flexible feed architecture: AT Protocol / Bluesky.
Both represent serious, principled attempts to build social infrastructure that isn't controlled by a single corporation. Watching how they evolve — and potentially interoperate — is one of the most interesting stories in the open web space.