With the launch of Threads, there's been a lot of interesting talk about the safety and privacy risks it poses to people on the fediverse, if and when Meta begins federating. But it's also worth examining the risks to the social systems.
First, some background
Kbin and Lemmy are link aggregators. They work a lot like Reddit: users uprank or downrank posts and replies, and this mostly determines what content ends up having the most visibility. Reddit calls this upvoting/downvoting, and that's useful terminology - at the social level of the app, no one user has significantly more power or influence than any other. There's a certain level of democracy in this type of model.
But if Meta's Threads ends up federating with Kbin & Lemmy, using the ActivityPub protocol they have in common? Things could get a lot less egalitarian.
Threads Breaks Kbin & Lemmy
For example: let's say a Lemmy user has some followers on Threads. Suppose the Lemmy user posts something about a celebrity's new diet pills. Maybe one of those Threads followers replies, and tags the diet-pill celeb account. The post gets boosted by the celeb, and appears in the feeds of their millions of Threads followers. Even if only a small fraction of those accounts boost the post, it still easily becomes the all-time top post on Lemmy.
Now imagine things like this happening all the time. The Kbin/Lemmy rankings stop being egalitarian. A large account on Threads can instantly make a post show up in millions of people's feeds, and make it much more likely to get boosted. Your front page is now filled with posts about diet pills and limited edition sneakers.
But it gets even worse than this: Meta's algorithm is part of this dynamic too. After all, they're the ultimate arbiters of what appears in Threads users' feeds. If the algo pumps posts with certain keywords to their users' feeds? Kbin & Lemmy don't stand a chance. You'll see whatever Meta's algorithms want their users to see.
Interoperability - it ain't all it's cracked up to be
Let's break down what just happened there. Both Lemmy/Kbin and Threads are using the ActivityPub protocol to communicate, making them interoperable. Post some text in one, you can read it on the other. Boost in one, that boost shows up in the other. And so on. The standard narrative here is that interoperability is a noble and worthwhile thing in and of itself; walled gardens are opened up, new possibilities for collaboration/communication/creativity emerge, and everyone is happy and free.
Sounds nice! But clearly, some apps don't work very well when they're slapped together with others. The protocol does exactly what it's supposed to, but the applications are so much more than just the protocol. They're also systems that have social dynamics and power relations baked into their design, and even though none of the users in this example are trying to game the system - although they definitely could - Threads still makes the Kbin/Lemmy ranking systems useless. And if the ranking systems are useless, then what's the point?
The Point
Put another way: the computers can talk to each other just fine. But for our human purposes, at the social level, these two applications aren't really interoperable at all. Not all so-called interoperability is desirable or beneficial, and 'federation by default' shouldn't be assumed to be desirable or beneficial either. What's more, Meta is a monopolistic bad faith actor. Defederate Meta. Nothing good will come of them being in your network.
(see also part 2: Threads undercuts Mastodon's core mission with 'algorithm creep': https://mstdn.patatas.ca/@smallpatatas/110707812951733786)
A few things here.
The first one that comes to mind is that defederation DOES stop your posts from going to Meta's platform when combined with the AUTHORIZED_FETCH server setting, while a simple user-level block may not. Depending on your server's settings, your posts may or may not be available on the open web where Meta could scrape the data - but this is still very different from them appearing in the feed or search results of, say, the transphobic, racist, or antisemitic groups that call Meta home.
This has serious implications for user safety and should not be overlooked. In fact, user safety is one of the biggest issues I have seen people mention when advocating for defederation.
Second: it's not yet clear if threads will allow their users to follow people on Lemmy or Kbin servers. But if they do, their users - including, for instance, the millions of followers of some big celebrity or politician - would be able to uprank posts and influence what you end up seeing. You might have LibsOfTikTok tell their users to brigade any posts critical of them, who knows. Meta's own algorithms could end up surfacing certain posts to their users, making the post rankings here largely a reflection of what Meta wants their users to see.
In other words, there's a lot more to the story than just 'blocking their content' when it comes to why you would want full defederation.
Here are a couple of blog posts that go into more detail around some of the data & privacy issues with federation:
https://privacy.thenexus.today/just-blocking-threads-isnt-enough/ discusses why defederation is much better than user-level blocking when it comes to protecting yourself from Meta
https://www.cacherules.com/blog/2023/6/resistance-is-futile-you-will-be-assimilated-by-meta/ discusses the things that Meta can learn about you via federation that they can't otherwise.