This post is part of a series on Mastodon and the fediverse. We also have a post on privacy and security on Mastodon, why the fediverse will be great—if we don't screw it up, and how to make a Mastadon account. You can follow EFF on Mastodon here.

A wave of people have announced that they're leaving Twitter to check out something called Mastodon, and that leaves many wondering, what is Mastodon anyway? More importantly, what is the “fediverse” and what is “ActivityPub”? This explainer will help you make heads or tails of this new approach to communications and social media.

What are the Fediverse, Federation, and Mastodon?

Federation is a broad term that means a group that has smaller groups within it which retain some measure of autonomy within that whole. In internet terms, the most well-known federated system is our old friend, email.

No matter how much you love or hate email itself, it is a working federated system that’s been around for over a half-century. It doesn’t matter what email server you use, what email client you use, we all use email and the experience is more or less the same for us all, and that’s a good thing. The Web is also federated – any website can link to, embed, or refer to stuff on any other site, and in general, it doesn’t matter what browser you use. The internet started out federated, and even continues to be.

The World Wide Web Consortium (W3C, a standards organization that gives us many protocols, especially HTML) created a protocol in 2018 called ActivityPub that enables federated systems similar to social networking. The systems built on top of ActivityPub, are collectively referred to as the fediverse.

One of the most famous services within the fediverse is Mastodon, a Twitter-like social network and communications system to which many users are switching following the recent turmoil at Twitter. At a very basic level, Mastodon is a web server (or app) that acts as a social network. Just like a service such as Twitter or TikTok, you use it by visiting a website or using an app on a smartphone, and you can post text, images, and videos that can be seen by your followers. You can also follow others and see their posts in your own timeline. In this way, Mastodon is very similar to services you already know and probably use every day. In fact, it’s very much like Twitter itself, which is why people unhappy with Twitter are considering Mastodon as an alternative and why we’re writing this essay. What makes Mastodon interesting, though, is that the server (or “instance”—the terms are interchangeable) that you might use isn't the only server running Mastodon in the world. 

Over the Garden Wall

In the early days of the internet, there were a number of contained services. America Online (AOL), Prodigy, and others were available to anyone who could access them before the internet proper was open to everyone. Those old systems had many of the same tools and services that we use today – instant messaging, email, shopping, and so on. The problem with those was that if you wanted to send a message to someone on another service, there weren’t good ways to do that. For example, AOL mail did not always allow someone to send an email to a person using Prodigy. The term “walled garden” was coined to describe the situation of these services in opposition to the internet itself.

The walls in these gardens eventually opened up, usually by using some underlying open, interoperable, standard protocol. For example, SMTP is the protocol we still use to send email from one system to another. HTTP itself is an open, interoperable way to get a web page as opposed to each service having a different way to construct and display a page. Now in that tradition, some are hoping ActivityPub will do the same for walled gardens in social media.

An Ecosystem built on ActivityPub

Underneath this, Mastodon is just one of a whole host of different services that communicate using ActivityPub. From one Mastodon server, a person can follow and be followed by anyone else on any other Mastodon server anywhere else in the world–just like you can send an email from one server to anyone else on any server in the world. ActivityPub is also able to convey many types of content, including text, pictures, and videos, but also concepts such as "likes," replies, and polls.

In fact, ActivityPub is so flexible that it forms the backbone for a number of diverse services in addition to Mastodon: PeerTube, a social video hosting site, like YouTube; PixelFed, which focuses on images; and Bookwyrm, a book cataloging and review site, similar to Goodreads. There is even an open source food delivery system that uses ActivityPub. The power of ActivityPub means that you can follow an account on any of those services even from one of the others. 

Each of these different services is a piece of open source software, so starting up a new server with the right software can immediately let you interact with all of the other servers out there. If you don't have the know-how or desire to maintain your own server, there are tons of public servers out there where you can create an account for yourself and interact with any user on any other server running any of those services. There are also many hosting sites that will do the heavy lifting of running a server on your behalf and using your domain name so that you can have your own ActivityPub services under your control.

This idea that people can interact with each other across servers is often the hardest piece for people to wrap their heads around, but, as we mentioned earlier, it works very similarly to email. Anyone who's ever seen an email address before knows that they have two parts: the username before the @ symbol, and the domain name after it. That domain name tells you what server that particular account lives on. Some people have email accounts with their university or employer, some have them with a public service like Google's Gmail, Microsoft's Office365, or Protonmail, but no matter what domain comes after the @ sign, you can always send a message to your mom, your friend, or your bank. That's because all of those servers speak the same protocol (called the Simple Mail Transfer Protocol) under the hood.

There is no limit to what services can connect to this growing network. Facebook and Twitter themselves could join the fediverse by implementing a suitable protocol on top of ActivityPub and thus sending their content out to the universe of federated ActivityPub servers and users.

Roadmap to the Fediverse

An account in the fediverse, such as on Mastodon, resembles email in that instead of everyone being on just one server (, there are many servers. Instead of someone’s handle being simply @alice, they might be There are already thousands of different sites that offer free Mastodon accounts, and just like email or web servers, you could run one yourself. There is even a website to help you pick which Mastodon instance you might like.  

This leads to the single biggest question people often ask when they approach ActivityPub, especially those moving to Mastodon from Twitter: "WHICH SERVER??" 

Fortunately, there are two good reasons to hold off on panicking. First, anyone on (almost) any server can follow and be followed by anyone on any other server, so you won’t be cut off from your friends and family if they end up on a different server. Second, the fediverse has mechanisms for moving accounts between servers, including ways to export and import your posts and follow and block lists as well as redirect your profile from one server to point to another. So, if for whatever reason you find you don't like the first server you land on, you can always move later.

That said, there are reasons to pick one server over another. The biggest one is moderation. Fediverse services are good at giving individuals the ability to block other accounts or even entire servers that they don't want to see in their timeline. They are also good at letting servers block accounts or entire other servers that don’t meet with their own moderation decisions. One could, for example, make an instance that only allows incoming posts that contain the word “cat” and permanently blocks anyone who uses the word “dog.” Thus, finding a server where you agree with the moderation policies may be a good idea. 

Another reason to pick one server over another is if that server is organized around a common community, maybe through a shared interest or language, and thus will have more conversations pertinent to that community. If you were involved in “Law Twitter” or “Infosec Twitter” or “Historian Twitter” then that might be a reason to pick one server or another. There are also special interest servers, like one made for present and past employees of Twitter.

What’s Different in the Fediverse?

While many people are moving from Twitter to Mastodon, let’s be clear: Mastodon is not the whole fediverse and the fediverse is not simply a Twitter replacement.

The fediverse is an example of how we can have a paradigm shift in how we do social media. It is still undergoing some growing pains—like a small town that is now seeing boom times. New people arriving in large numbers can change the trajectory of the social part of the network, and come with their own points of friction.

Twitter, as we know it today, has developed over fifteen years and has seen many changes and emergent features–both Twitter hashtags and at-signs were invented by the users, for example. It has also had a dedicated team of professionals build it. Today’s fediverse services are a labor of love from a group of software communities.

Fediverse software is not as robust as Twitter software yet; as one might expect from a decentralized system, there are also several common clients with their own features and issues. The support for multiple accounts is still somewhat spotty, for example. Many small features a Twitter user is accustomed to may not be built yet. On the other hand, these services have many features Twitter users have been asking for, such as higher character limits, content warnings, and an option to automatically delete old posts.

The fediverse has no central authority and that means some features like Twitter’s original blue-check verification simply don’t exist. The closest thing to getting “verified” is proving to your instance that you control an external webpage or resource by including a special hyperlink to your profile.

Since the fediverse is decentralized, there is no single authority to moderate posts, or remove accounts – that’s left to the users and servers themselves. Mastodon users typically mark posts with content warnings, not only for genuine sensitive content (e.g. content warning about war news), but also to minimize a post’s footprint on your timeline. In conjunction with hashtags, they are also used for categorizing and curating posts that are not sensitive (e.g. content warning: “My cat #pets”).

However with these differences, one could easily turn the question on its head– what should mastodon users know about switching to Twitter? There will always be tradeoffs evolving between incumbent social media and their federated alternatives. This new spark of competition between platforms and federations holds the potential for new innovations and improvements to our autonomy online.