Meta—the company behind Facebook and Instagram—announced that Threads is joining the Fediverse. That is, a Threads user can follow a Mastodon user, and vice versa. Further, you may have noticed that Mastodon is just not a single service; it's a collection of different service all using the Mastodon software, but each instance can still communictae with each other.

This is all possible because Mastodon implements—at least partially—the ActivityPub and WebFinger protocols, and soon Threads will do so, too.

With ActivityPub and WebFinger, as long as you know the account address of a particular user, then you can interact with that user, even if they are not on the same server as you are on.

This is the power of federation.

This runs contrary to what has traditionally been done; previously, when you signed up for a social media service, and you wanted to get in touch with a friend, that friend needs to have had also signed up for the service, otherwise the two of you will need to find an alternative arrangement.

This is not anything unique. This idea has been around since at least as far back as the invention of email.

In the case of ActivityPub—along with WebFinger—both you and that friend can still communicate, as long as both services use the same protocols.

What can ActivityPub do?​

ActivityPub has explicitly defined support for the following actions that are often expected on many social media platforms:

• Follow
• Block
• React to an object with a "like"
• Create a post
• Delete a post
• Update a post
• Share another post (via an announce)
• Undo a previous action

Can ActivityPub do more?​

Developers can extend what AcivityPub is capable of.

That said, any extensions implemented in a Fediverse software needs to be supported in other such software, in order for that extension to be fully comprehensible.

Fortunately, software that don't offer support for such extensions can fallback to showing merely what it does understand so far, as well as include a link to the original content.

What else is ActivityPub useful for?​

ActivityPub is based very heavily on the concept of an actor, and employs a pattern called the "actor model". In the actor model—as used by ActivityPub—an actor can send messages to other actors.

Actors are often associated with an account, but don't always need to be. An actor can be a bot that merely reposts content not explicitly on that platform the bot is posting on. This is especially useful for services that expose an ActivityPub endpoint.

In other words, an ActivityPub actor doesn't need to be a user.

A good example of this is an ActivityPub group actor.

With a group actor, you can implement the concept of a group, where people can post to it by sending a note to a group. Once a note has been posted, the group will boost that note to anyone who follows that group.

Both /kbin and Lemmy heavily use the ActivityPub group actor.

In Summary​

ActivityPub is what enables a decentralized social media experience, without requiring everyone to be a part of a single entity—typically a corporation.

With ActivityPub, you have an actor that you can address, in order to send them messages, and that same actor can respond to the actor associated with your account with messages as well.

ActivityPub is essentially like email, but with added features that allow you to send not only messages, but also follow others, be followed, and broadcast to your followers.

With the new API pricing announced by Reddit, apps like Apollo—a third-party Reddit client—has now been slated to shut down on June 30th, due the financial infeasibility of allowing users to continue using it. Users and moderators—who are often managing communities for free—have gone on a site-wide protest against changes introduced, among other reasons.

As reports of an exodus from Reddit unfolds, software like Lemmy and /kbin, have been gaining popularity. In fact at the time of writing this, Lemmy has grown to become the fifth most popular software on the Fediverse.

What are Lemmy and /kbin?​

According to the official Lemmy website:

Lemmy is a self-hosted social link aggregation and discussion platform. It is completely free and open, and not controlled by any company. This means that there is no advertising, tracking, or secret algorithms. Content is organized into communities, so it is easy to subscribe to topics that you are interested in, and ignore others. Voting is used to bring the most interesting items to the top.

And, according to the official /kbin website:

/kbin is an open source reddit-like content aggregator and microblogging platform for the fediverse.

Create and moderate communities, meet people with similar interests, and develop your passions.

In other words, both Lemmy and /kbin bring Reddit-like features to the Fediverse.

With support for WebFinger and ActivityPub, users of Mastodon, Calckey, Friendica, or any other ActivityPub and WebFinger software can easily follow Lemmy and /kbin users.

However, a feature that Lemmy and /kbin have, that Mastodon does not, are groups.

Joining groups from platforms that don't support groups​

So you haven't joined neither /kbin or Lemmy yet, and hesitant to join either, but would still want to join a group on the Fediverse?

There's good news! A group in Lemmy and /kbin is just another ActivityPub actor.

Mastodon, Calckey, etc. are all implementations of ActivityPub. ActivityPub has the concept of an actor. Both users and groups are actors, and because actors can follow other actors, this means a user can follow a group!

Following a group has a very similar procedure to following a user.

When visiting a /kbin or Lemmy Group page, copy the adress from the address bar, and paste it into the search bar of your Fediverse software. You should see the group show up as an entity that you can follow.

Congratulations, you have now joined a group!

Once joined, you should see posts being reposted (a.k.a. boosted) by the group that you "followed" onto your timeline.

To post into the group, your post should tag the group's account address.