ActivityPub
Example
The Use of ActivityPub by Mastodon, 2026
Connected Places:
"The simplest way to understand ActivityPub is that servers send messages to each other. A person on one server can follow a person on another server, and when either of them posts something, their server delivers that message to the other. That simplicity hides a deeper question though: what is a server actually for?
The answer that dominated for most of Mastodon’s history is that a server is infrastructure. In a 2023 interview with The Verge, Mastodon founder Eugen Rochko described the product in terms that made servers sound like an implementation detail. “Think about it like email, and you’ll get it. If you don’t like Gmail, you can switch to something else, but you don’t have to quit email entirely as a concept.” When users complained about having to choose a server at signup, Rochko’s response was to make the choice disappear: sorting the server list to show larger servers first, adding a “pick for me” button, and keeping mastodon.social open as a default funnel. He acknowledged this moved Mastodon closer to the Gmail problem he said he wanted to avoid, but framed it as necessity, saying: “However, I’ve learned over the years that there is no replacement for having a default, right?”
In this model, the server boundary is administrative rather than social. Users experience a single network where the fact that their account lives on one server rather than another is, ideally, something they never have to think about, with the server handling uptime, storage, and moderation enforcement. The social graph extends freely across server boundaries and the home feed pulls in content from everywhere, making it feel like a single network.The topology is functionally centralized even though the infrastructure is distributed. This solves some problems regarding engineering and the distribution of power, but does so without creating distinct social spaces.
Mastodon’s current leadership is trying to move away from this. Their new community director, Hannah Aubrey, describes Mastodon as “a front door, not the whole house,” and talks about surfacing and supporting other servers rather than funneling everyone into mastodon.social. Director Felix Hlatky has made distributing users away from mastodon.social an explicit priority, noting that concentrating users on one server is “not the purpose of building a social network.” The target is ambitious: going from roughly 10,000 servers to 100,000, which Hlatky says “needs a mindshift in what it means to start a server.”
In this newer model, the server boundary becomes a social boundary, and your experience of Mastodon is supposed to be shaped by the community you joined rather than just by who you follow across the wider network. Aubrey envisions server starters as “leaders and organizers” who want to build something for their community, whether that community is defined by language, identity, geography, or shared interest, framing the value proposition in terms of belonging: “you can still be in community with people in a safe and healthy way.”
Instead of one network with distributed infrastructure, it is a network of networks where each node has meaningful social coherence. Newsmast, a UK-based organization building ActivityPub infrastructure for publishers and communities, is pushing this further by creating branded apps on top of individual server communities that combine community feeds with a publisher’s content. The logical conclusion of their approach is one app per community, with federation as the connective tissue between them.
Lemmy and PieFed, the Reddit-style link aggregation platforms on ActivityPub, complicate this further. Communities (the equivalent of subreddits) exist on servers but are accessible from other servers, creating a double layer of social boundaries: a community has its own topic, its own moderators, and its own norms, but it also lives on a server that has its own administrators and its own moderation policies. When users from one server participate in a community hosted on another, it is genuinely unclear whose norms apply. The server boundary and the community boundary overlap without aligning, producing a topology of mixed jurisdictions that nobody has fully figured out how to govern.
(PieFed also has a feature called ‘Topics’, which aggregates posts from multiple communities into a single feed around a single theme, making jurisdiction a truly joyful mess: a single post can be made by someone on server A, posted into a community hosted on server B, and then aggregated into a Topic hosted on server C. What happens when rules between servers A, B and C conflict is anyone’s guess.)
The point here is that ActivityPub does not prescribe a single topology. The protocol is flexible enough that each application makes a fundamentally different choice about how much the server boundary matters socially. Mastodon circa 2023 treated servers as invisible plumbing; Mastodon in 2026 wants servers to be communities. PeerTube treats them as institutional containers for self-hosted video libraries, with federation as a secondary feature. Lemmy layers communities on top of servers, creating overlapping boundaries."