In the graph above I’ve arranged the 28 Threadiverse instances with at least 200 monthly active users (MAU)—orange instances running Lemmy, green Piefed, and red Mbin—on the basis of which instances they defederate.
Each instance has its MAU count listed below it, sourced from the latest MAU figures at Fediverse Observer, with adjustments made to the totals for Piefed (due to undercounting by outdated instances, as noted here; I used @Vicinus@piefed.zip’s suggested estimate of 7,200 Piefed users) and Mbin (due to tardigram.com having a duplicate listing).
After adjustments, there are an estimated 43,287 monthly active Threadiverse users (35,184+7,200+903), of whom 87.84% are on the 28 largest instances. The percentages listed alongside the MAU figures correspond to each instance’s share of the total MAU count.
Defederation data was sourced from Federation Checker. The tool doesn’t support Mbin instances, so I left fedia.io in its own category. If any defederations are missing, feel free to let me know. Ideally such a chart should also include community blocks to better illustrate which instances block what, but that data doesn’t appear to be as easily searchable.
I also noticed cases of some instances being listed as being defederated by another instance, despite not appearing on the defederation list for that instance (i.e. reddthat.com and lemmygrad.ml are listed as being mutually blocked, despite neither having the other on their block lists). In testing that federation, I noticed that !wholesome@reddthat.com did not federate to Lemmygrad while !memes@reddthat.com did, so clarification on that matter would be helpful.
While I’ll update my generalized chart as needed to correct errors, perhaps at some point an automated federation mapping tool similar to the one feddit.de formerly hosted could be developed to more completely illustrate instance federation.


That’s not true at all, the maintenance costs of lemmy.ml’s are miniscule compared to what the devs get.
As I understand it, keeping in mind it’s been a bit since I looked at it, it depends on where the community the interaction is taking place in is hosted. That’s always the central truth and where other instance collect the votes, comments etc. from. So say piefed user Alice blocks a user Bob who is on a different instance, or the instance Bob is on altogether.
If Bob then replies to Alice in a community hosted on lemmy then the reply is accepted and federated out to everyone. Alice’s instances notes that she blocked Bob and drops the replies, no one on Alice’s instance can see them.
If Bob replies to Alice in a community hosted on Alice’s instance then Bob’s instance logs the reply, sends it to Alice’s instance where it gets discovered that Alice blocked Bob and the reply is silently dropped. Any instance now checking the community for new comments will not receive Bob’s reply since Alice’s instance dropped it. Bob’s instance however already logged the reply, so it remains local to Bob’s instance only.
What I don’t know is what happens among piefed instances, I think there is some functionality to notify Bob’s instance that Alice blocked him if Bob’s instance is running Piefed and it will delete the comment, but that part I’m least sure about.
The way blocks work in Lemmy is that a user will simply not be shown what they block, federation is completely unaffected. An ordinary user can only decide what they are shown not what others are shown.