Computing Applications

After Twitter

It is not yet clear which platform, if any, will replace Twitter in popularity, but a number of applications are making a bid.

When Elon Musk took over Twitter in October 2022—laying off half the staff, altering the blue checkmark verification scheme, reducing content moderation, and reinstating previously banned users—many of its users started casting about for alternatives. One social network, an open-source platform called Mastodon, reported that it grew from 380,000 active monthly users before Musk's takeover to 2.5 million in November and December of that year, though that number has since dropped off.

It is not yet clear which platform, if any, will replace Twitter in popularity, but a number of applications are making a bid. Some were created well before the Twitter meltdown, while others have cropped up since. Some are taking a decentralized approach that they say will be better for user choice, while others are sticking with a self-contained model.

Mastodon is operating on the decentralized model in what is known as the federated universe, or "fediverse." In the fediverse, no one group controls a platform, and different applications can easily interact with each other. A Mastodon user could, for example, find and view content posted on PeerTube, a video hosting site. Anyone who wants to could start their own server node, which Mastodon calls an "instance," with its own rules, moderation policies, or user limit. "There are hundreds, maybe thousands, of single-person instances, where people just set up their own server on a cloud service or on a Raspberry Pi [single-board computer] in their home, and they are perfectly able to manage their own social network," says Evan Prodromou, a software developer who co-created ActivityPub, the protocol on which such applications are built.

The World Wide Web Consortium (W3C) adopted ActivityPub as a protocol for federated servers in 2018. It provides a client-to-server application programming interface (API) that allows users to create, update, and delete content, and federated server-to-server API for distributing that content and delivering notifications.

ActivityPub dates back to Prodromou's early work in federated social networks, when he created an open-source social networking service called, which he says resembled Twitter. To run it, he and co-developers created a federated standard called OStatus, which became the basis for the microblogging software StatusNet, which in turn evolved into GNU Social. Another standard developed around the same time was Activity Streams for data representation, written in the programming language Atom and using extended markup language (XML). In 2017, Prodromou helped moved W3C to Activity Streams 2.0, which replaced XML with Javascript Object Notation (JSON), making it more extensible. Adding ActivityPub provided the server interfaces.

GitHub lists 30 services using ActivityPub federation, including microblogging services such as MissKey, a social network for talking about books called Bookwyrm, an image-sharing service called PixelFed, and several that focus on podcasting. "There are some interesting dynamics that happen when you have a federated network," Prodromou says. "It becomes a stronger network when you have a lot more players."

There are some downsides to the federated model, however. For one, with no central hub, casual users might find joining a network like Mastodon daunting. They sometimes struggle to figure out which instance to join, as they all have different names and different moderation policies, and if someone running a server decides to shut it down, or a user is banned for violating its policies, all the posts and contacts that person has developed could disappear. That last concern led William Casarin, a computer software engineer in Vancouver, Canada, to start building Damus, a Twitter-like social network for iPhones built on iOS and based on NOSTR, an open-source, decentralized protocol similar to ActivityPub. NOSTR stands for "notes and other stuff transmitted by relays" and was created by someone who goes by the pseudonym @FiatJaf.

Casarin was annoyed when posts that he thought were innocuous, such as talking up Bitcoin, got him banned from a Mastodon instance. The main difference in NOSTR, Casarin says, is that messages and contacts exist as "little JSON blobs" that are signed by an encryption key the user controls. Whereas ActivityPub links user data to whichever server they signed onto, NOSTR sends the blobs to perhaps 10 different servers. "If you're ever banned from one of the relays, it doesn't matter because then you can sync it to other relays. So you have much more control over your social graph and no one can take it away from you," he says.

That could make applications using NOSTR more attractive to the people who worry that content moderation can lead to censorship. NOSTR would allow for a more decentralized moderation, where different communities of users could make decisions about what is and is not allowed, he says.

NOSTR has attracted a lot of Bitcoin users, Casarin says, because it allows people to make micropayments through the Bitcoin Lightning Network, though that is not necessary to build an application. It has also attracted Jack Dorsey, former CEO of Twitter, who last year donated 14 bitcoin—worth about $245,000 at the time—to NOSTR. While he was still at Twitter, Dorsey also founded Bluesky, with the aim of creating something similar to Twitter but federated.

Bluesky is building its open source Authenticated Transfer Protocol (ATP). Like NOSTR, it uses public-key cryptography to authenticate data. It allows users to move from one hosting server to another without losing their data, and gives them more control over what appears in their feed than Twitter does. The company started rolling out a beta-test version in February, gradually ramping up the number of users by occasionally handing out invitations that existing users could share with a friend. As of early June, Bluesky said it had more than 100,000 users.

Company officials from both Bluesky and Mastodon declined requests for interviews. A blog post on Bluesky's website says, "Our priorities continue to be a focus on moderation and curation to ensure a safe and pleasant environment for users, and protocol work to launch federation." Casarin says the architecture of ATP looks similar to that of ActivityPub, but it lacks the interoperability between apps that NOSTR provides. "It's really just focused on a Twitter-like use case, so it's much more limited in scope," he says.

Some Stand Alone

Not everyone is pursuing federated social networks. T2 was founded in November 2022 by former employees of Google, Twitter, and Discord—a voice over Internet Protocol and instant messaging system popular with gamers—not long after the Twitter meltdown. It started issuing invitations to join in late April, passing 3,000 users at the beginning of June.

Gabor Gselle, co-founder and CEO of T2 (a name he hopes to replace), says safety and trust are easier to maintain in a non-federated system. In Mastodon, for example, if one instance has policies the owner of another instance dislikes, they simply block access to that entire instance. "It becomes the opposite of what the goal was of the Federation movement," he says. "It's a balkanization of these social networks where the nice instances don't talk to the naughty instances and you have an echo chamber in in each one of them."

Whereas efforts at content moderation were built into the previous generation of social networks after they had already existed for some time, he says, T2 is focusing on that early, including tools to automatically detect problems like profanity, harassment, and threats in both text and images. That is not as challenging a task as when he was at Twitter several years ago, he says, because commercially available APIs now contain technology to easily catch a lot of the problems.

In fact, he calls the technology behind T2 pretty vanilla. The site runs on Amazon Web Services and uses an off-the-shelf open source database, allowing it to avoid some of the scaling difficulties Twitter ran into early on. "Moore's Law has helped us to do this in a much simpler way than it had to be done 17 years ago," Gselle says.

The ready availability of pre-existing code, along with websites that teach people how to code, helped Raluca Pop, who also goes by Kassandra, when she set out to launch Hive Social, a stand-alone mobile-only app, in 2019. A psychology student at California State University, Long Beach, she didn't like some of the changes she was seeing in her social networking apps. Instagram, for instance, changed its algorithm so the pictures people posted did not necessarily go out to all their followers. She used the website Udemy to teach herself the Swift and XCode programming languages, designed for creating apps for iOS. She has since changed the code to Flutter, which also accommodates Android phones. The company now has five employees.

The app is primarily media-focused, for posting photos, videos, and text, and also allows users to customize it. "We wanted to take great care of our UI and UX design and we've had a lot of positive feedback about it and we continue to improve it," Pop says.

In the end, though, it may not be the technological details that make one platform rise above the others. Meta, the company that owns Instagram and Facebook, is reportedly working on a federated text-based app for Instagram, though it has said little publicly about the project. Jen Golbeck, an expert in social media at the University of Maryland, said such as app could leave others in the dust simply due to the network effect; the simple fact that so many people already have a presence on Instagram makes it enticing to other users, who want to be where everybody else is.

Golbek currently runs a Twitter account about rescued golden retrievers that has 165,000 followers. "I can push a button and those 100,000 followers are now going to be following my Instagram Twitter-clone account. There's nothing any other platform could offer that's as powerful as bringing 100,000 people with me," she says.


Neil Savage is a science and technology writer based in Lowell, MA, USA.

Join the Discussion (0)

Become a Member or Sign In to Post a Comment

The Latest from CACM

Shape the Future of Computing

ACM encourages its members to take a direct hand in shaping the future of the association. There are more ways than ever to get involved.

Get Involved

Communications of the ACM (CACM) is now a fully Open Access publication.

By opening CACM to the world, we hope to increase engagement among the broader computer science community and encourage non-members to discover the rich resources ACM has to offer.

Learn More