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the latest MAU figures at Fediverse Observer, with adjustments made to the totals for Piefed (due to undercounting by outdated instances, as noted here; I used @Vicinus@piefed.zip's suggested estimate of 7,200 Piefed users) and Mbin (due to tardigram.com having a duplicate listing).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.
Unless an instance chooses to defederate from another, everything is federated by default; indicating that with arrows would result in a very messy graph.
Fedinsfw.app, like lemmynsfw.com before it, is an instance dedicated to hosting NSFW content. As such content can be off-putting for new users, and only a subset of users are interested in seeing it, some instances choose to defederate from instances dedicated to it. While I'm broadly against defederation that results in a fractured Threadiverse, in this case doing so is probably more helpful in terms of new user retention than it is harmful (users can easily have separate accounts on SFW and NSFW instances or choose an instance that maintains federation with both).
This is not always true, as Hexbear shows. Whether an instance defaults to federation or defederation is an admin setting.
I realized that afterwards as people replied; in reviewing all instances with at least 50 MAU, I found that while Hexbear is the main user of the whitelist system, lemmynsfw now also uses it for exclusive federation with fedinsfw.
Lemmy.sinistraverso.org has it enabled, but doesn't have any instances on the list, so it doesn't seem to do anything in its case.
I think the bigger reason for fedinsfw's defederation, at least in terms of our instance and Blahaj, is the fake CP. Might be the case for other instances in the black box too.