Privacy

Federation is less friction imo. If you want to message someone who has a signal account, you have the friction of making yet another account to message them. If you then want to message someone who uses whatsapp, you need to make yet another account to message them. Etc, etc. That seems far worse than the awkwardness of trying to interact with microblog activitypub platforms within threadiverse activitypub platforms. Granted, you still have multiple federation standards (ie: email, activitypub, matrix) that don't directly interact at all (although that might be for the best).

With federated platforms, you can message anyone using the same federation standard. Unfortunately, there's more than one, so you need an email address to message anyone else who uses email regardless of whether they're icloud or google or aol or self-host. But you can't email someone using activitypub or matrix directly. And even within activitypub, its far from seamless to interact between different parts (piefed/mbin/lemmy all work pretty well together, but interactions between more different parts because more awkward).