Decentralized Social Media Models

• Technical Focus and Design: ActivityPub, used by Mastodon, emphasizes federated networking with an Activity-Object model in JSON-LD, enabling interoperability across platforms like PeerTube and PixelFed. In contrast, the AT Protocol, powering Bluesky, prioritizes account portability through Decentralized Identifiers (DIDs), algorithmic transparency, and scalability via aggregated activity feeds.

• Interoperability and Ecosystem: ActivityPub supports a broad ecosystem within the Fediverse, fostering interaction across diverse platforms. The AT Protocol, while still maturing, focuses on portability and customization but is currently centered around Bluesky as its primary implementation.

• User Control and Scalability: ActivityPub allows users to select servers with independent governance but offers limited tools for account migration. The AT Protocol simplifies data portability and scalability by using signed repositories and aggregated feeds, though this partially centralizes activity aggregation.

Decentralized social media platforms like Bluesky and Mastodon have brought attention to the protocols that power them: the AT Protocol and ActivityPub, respectively. Both protocols aim to challenge traditional centralized social media by emphasizing user control, data ownership, and interoperability. However, they differ significantly in their technical design and social implications. ActivityPub, widely adopted within the Fediverse, supports federated networks with instance-based governance and interoperability across diverse platforms like Mastodon and PeerTube. In contrast, the AT Protocol, used by Bluesky, focuses on account portability via Decentralized Identifiers (DIDs), algorithmic transparency, and scalability through aggregated activity feeds. These differences highlight unique approaches to decentralization, offering users distinct options for reclaiming control over their digital social experiences.

1. Core Technical Differences

ActivityPub, the protocol behind Mastodon, is an open standard designed for federated social networking. It uses an Activity-Object model in JSON-LD format to structure data and supports a wide range of actions like creating, updating, deleting, following, liking, and reposting content. ActivityPub is widely adopted across the "Fediverse," a collection of interconnected servers (instances) that allow users to communicate seamlessly while maintaining independent server governance. This federated approach emphasizes openness and content sharing across platforms.

In contrast, the AT Protocol, used by Bluesky, focuses on enhancing account portability and algorithmic transparency. It employs Decentralized Identifiers (DIDs) for user identities and uses signed data repositories to ensure users can migrate their accounts between servers without losing their content or social connections. Unlike ActivityPub's reliance on JSON-LD, the AT Protocol uses lightweight JSON schemas with its custom tool called Lexicon, which simplifies schema creation for developers. Additionally, Bluesky's AT Protocol centralizes some aspects of activity aggregation to reduce server load and improve scalability.

2. Interoperability and Ecosystem

ActivityPub excels in fostering a diverse ecosystem of applications beyond Mastodon, including platforms like PeerTube (video hosting), PixelFed (photo sharing), and WriteFreely (blogging). Its widespread adoption has created a robust network where users from different platforms can interact without barriers. This interoperability aligns with its mission to decentralize social networking while maintaining a high degree of openness.

The AT Protocol is designed with interoperability in mind but has yet to achieve the same ecosystem diversity as ActivityPub. Currently, Bluesky remains the primary implementation of the protocol, limiting its reach compared to ActivityPub's expansive Fediverse. However, Bluesky's emphasis on algorithmic customization allows users to control how content is curated and displayed, offering a more personalized experience than ActivityPub-based platforms.

3. User Control and Data Portability

Both protocols prioritize user control but approach it differently. ActivityPub allows users to choose their server (instance) within the Fediverse, granting them autonomy over moderation policies and data storage. However, migrating accounts between servers can be cumbersome due to limited tools for transferring historical data or redirecting followers.

The AT Protocol addresses this limitation by making data portability a central feature. Users can seamlessly move their accounts between servers without losing access to their posts or followers, thanks to its use of DIDs and signed data repositories. This ensures greater resilience against server shutdowns or policy disagreements—a key advantage over ActivityPub's migration tools.

4. Privacy and Security

Privacy is a shared priority for both protocols but is implemented differently. ActivityPub relies on instance-level governance for privacy controls, meaning that individual server administrators determine how user data is handled. While this decentralization reduces reliance on a single authority, it also introduces variability in privacy standards across instances.

The AT Protocol enhances privacy by giving users granular control over their data through cryptographic techniques. Signed data repositories ensure that only authorized parties can access user information, providing stronger guarantees against unauthorized use or breaches. Additionally, Bluesky's approach minimizes metadata exposure during interactions compared to ActivityPub's more open communication model.

5. Scalability Challenges

Scalability is a critical consideration for decentralized networks as they grow in popularity. ActivityPub's federated model distributes traffic across numerous small-to-medium-sized servers (instances), which can lead to performance bottlenecks when individual servers are overwhelmed by high activity levels or large user bases.

The AT Protocol addresses scalability differently by using aggregating applications that consolidate activity from multiple hosts into a unified feed. This reduces the load on individual servers while providing users with global views of activity across the network. However, this centralized aggregation raises questions about whether it compromises the decentralized ethos compared to ActivityPub's fully federated approach.

6. Censorship Resistance

Both protocols aim to resist censorship but take distinct approaches due to their architectures. In ActivityPub-based networks like Mastodon, censorship resistance stems from the decentralized nature of instances—users can migrate to another server if their current one enforces restrictive policies or shuts down entirely.

The AT Protocol enhances censorship resistance through account portability and cryptographic guarantees that prevent servers from tampering with user data or identities. By allowing users to retain full control over their accounts regardless of server policies, it offers an additional layer of protection against arbitrary bans or content removal.

7. Community Governance

Community-driven governance is integral to both protocols but manifests differently due to their structures. In ActivityPub networks like Mastodon, governance is decentralized at the instance level—each server sets its own rules for moderation and community standards based on user consensus or administrator decisions.

Bluesky's AT Protocol envisions governance mechanisms that prioritize algorithmic transparency and user choice over content curation. Users can select or customize algorithms that determine what content appears in their feeds rather than relying solely on platform-imposed algorithms—a feature that distinguishes it from both centralized platforms and ActivityPub-based networks.

8. Adoption and Ecosystem Maturity

ActivityPub has achieved broader adoption due to its early development and integration into multiple platforms within the Fediverse ecosystem. Its widespread use has created a mature network where users can explore diverse applications beyond microblogging.

The AT Protocol is still in its early stages of adoption, with Bluesky serving as its flagship implementation. While this limits its current ecosystem diversity compared to ActivityPub, Bluesky's rapid growth suggests potential for broader adoption as more developers build on the protocol.

The Final Verdict between AT Protocol and ActivityPub

The choice between Bluesky's AT Protocol and Mastodon's ActivityPub depends on individual priorities for social networking experiences:

• ActivityPub offers a well-established ecosystem with diverse applications and strong interoperability within the Fediverse.

• AT Protocol focuses on account portability, algorithmic transparency, and scalability improvements while still building out its ecosystem.

Both protocols challenge traditional centralized social media models by empowering users with greater control over their data and interactions. As decentralized social media continues to evolve, these protocols represent complementary approaches that cater to different user needs while advancing the broader goal of decentralizing online communication networks.