Afaik, whenever an Activitypub instance has defederated from another it has always had to do with some combination of bad user behavior, poor moderation, and/or spam. Are the various instance admins who have decided to preemptively block threads.net simply convinced that these traits will be inevitable with it? Is it more of a symbolic move, because we all hate Meta? Or is the idea to just maintain a barrier (albeit a porous one) between us and the part of the Internet inhabited by our chuddy relatives?
(For my part, I’m working on setting up my own Lemmy and/or Pixelfed instance(s) and I do not currently intend to defederate.)
Facebook can bootstrap their product with federated content made by users who are in the fediverse because they don’t want to support a company like Facebook. By not defederating, you would be helping Facebook every time you post a comment or make a post because you would be giving Facebook free content to further their for-profit goals.
Facebook will also be taking fediverse content and displaying it next to ads.
Defederation is one-way. An instance you defederated with still has access to your instances posts.