Threads is not a Lemmy instance, but it is implementing the ActivityPub protocol and thus will be able to federate with Lemmy instances, Mastodon servers, presumably kbin, Friendica, GNU Social, etc.

don’t lose your head over this. lemmy, kbin, mastodon and apparently threads (the new twitter alternative by facebook/meta) are all part of the fediverse, which means they all follow a decentralised approach. mastodon and threads are microblogging platforms, while kbin and lemmy have a similar format to reddit. because they are all part of the fediverse, all these platforms communicate with each other and you can use kbin to subscribe to microblogs such as mastodon and have them appear in your feed. defederation basically means cutting the link between one server and another, so they can’t communicate anymore.

Lemmy is a piece of software. Lemmy software is a link aggregator - same as reddit.

So you’re signed up to an instance of Lemmy which is installed on the server at lemmy.ml - this means that the server you signed up to (lemmy.ml) is running a copy of the Lemmy software. Other servers also run the Lemmy software making them also instances of Lemmy. As well as you being able to talk to users in Communities on the lemmy.ml server, you can talk to users in Communities on other Lemmy instances. For example, I’m registered on the server at lemmy.world

KBin is also link aggregator software, just like Lemmy and Reddit. Same things apply there, same software on multiple servers, all able to talk with each other.

Mastodon software is a microblogging service - same as Twitter (and Threads). Just like instances of Lemmy, instances of Mastodon can talk to each other. So a user on mastodon.world can talk to (for example) a user on kolektiva.social which is also running the Mastodon software.

Plume is blogging software - like WordPress, but just like Lemmy and Mastodon, it can be installed on multiple servers, all of which can talk to each other.

There’s also Pixelfed (Instagram), PeerTube (YouTube), Friendica (Facebook) and a large variety of others.

Now, as well as all these different types of software (Lemmy, Mastodon, KBin, PixelFed etc) being able to talk to other instances of the same software on other servers, because they are all underpinned by a single method of passing information called ActivityPub, each type of software can also talk to each other - so you as a Lemmy user can also see posts (like the one you and I are responding to) from a user on a server running an instance of Plume. Some people here are commenting from a Mastodon instance. All these things are loosely joined together making a joined (federated) universe - the fediverse.

What people call “the Fediverse” is a collection of web applications that talk to each other through an open standard called ActivityPub. ActivityPub defines what users, groups, posts, comments and likes are, how you can subscribe to them and how they travel between instances. People have built different software packages that all use ActivityPub but have different user interfaces to feel similar to different “traditional” platforms. Mastodon is like Twitter, Mastodon and kbin are similar to Reddit, PeerTube is similar to YouTube, Pixelfed is similar to Instagram or Flickr and Plume is a long form blogging platform similar to Medium or older versions of Wordpress. Because they all use the same protocol under the hood, they can generally talk to each other. The user experience isn’t great yet but you can already use your Mastodon account to post to a lemmy community or to comment on a Plume post. Imagine it a bit like email where Gmail’s web interface, MS Outlook, Thunderbird and dozens of other clients exist as well as several different Mail servers. They can all talk to each other even though they were written by different people and all have their own interpretation and extensions to the SMTP and IMAP standards that define how emails work

Threads is a new microblogging Application by Meta (Facebook / Instagram) that will probably work very similar to Mastodon. In contrast to most other fediverse applications, Threads won’t be open source but will still use ActivityPub so it will be able to talk to existing open source applications. People here are afraid that they will abuse that to spy on people or systematically archive everything that happens in the fediverse in order to sell your data or train AI with it. They propose that we defederate from Threads (meaning we block our instances from talking to Threads’ instance). My post contains my thoughts on why that isn’t as useful as people think it is.

Hope that helps. If you still have questions, I’m happy to answer them to the best of my knowledge.

Not my original intention but glad you liked it.