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the latest MAU figures at Fediverse Observer, with adjustments made to the totals for Piefed (due to undercounting by outdated instances, as noted here; I used @Vicinus@piefed.zip's suggested estimate of 7,200 Piefed users) and Mbin (due to tardigram.com having a duplicate listing).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.
Come to think of it:
I still remember when Lemmy was fairly new. Before $20M for the client API killed off all third-party Reddit apps, enshittifying Reddit to such a point that people left for the Threadiverse in droves in protest (and looking for something that didn't require either Reddit's Web interface or the official Reddit app). When /kbin was still experimental (as opposed to dead), when Mbin and PieFed didn't exist yet, but Lotide still did (although it was more or less a zombie).
Back then, federation was optional on Lemmy. As in, Lemmy servers weren't federated with anything by default. The admins had to manually add every single last Fediverse server, be it Lemmy, be it /kbin, be it something else, to an allowlist to allow it to federate with their server.
Lemmy servers actually had public lists of servers they federated with (as opposed to lists of servers they had defederated, available on third-party websites).
Not being federated with any given Lemmy server was the default back in the day. Most Fediverse servers were defederated from Lemmy servers because the admins of the Lemmy servers simply didn't know that they existed. If you were lucky, then the admins occasionally checked their server logs, discovered Fediverse servers that they had never heard about, checked them and then added them to the list.
It was only a while later that a new Lemmy version switched to federating by default, simply because the above was tedious and impractical.
As others informed me, Hexbear still uses the allowlist system. While it seems like they're pretty good about adding instances that don't defederate them to their whitelist, six instances with over 50 MAU that don't defederate them are effectively defederated from them due to not being on their whitelist.