User accounts are fragmented and just because you signed on at lemmy.world doesn’t mean your account exists on lemmy.ca.
https://github.com/LemmyNet/lemmy/issues/1985
Communities are fragmented and /c/games on lemmy.world is completely different than the one on lemmy.ml with its own users, set of posts, etc.
Lemmy does not currently allow for instance or user migration.
https://github.com/LemmyNet/lemmy/issues/3057
Nor does it allow for shared communities (ie the aforementioned /c/games is unified across multiple instances)
https://github.com/LemmyNet/lemmy/issues/3100
We are in the early days. If you’re eager feel free to join in the development on these any many other core issues. There’s real potential here.
I think this would have to start off as some kind of defined standard in ActivityPub before it could be implemented across all ActivityPub services and be interoperable.
Otherwise, it will result in fragmentation.
But yeah, having one ActivityPub account for many services is ideal IMO, along with being able to follow any content on your platform of choice (like interacting with Peertube or Mastodon postings in Lemmy, or even something rediculous like interacting with Lemmy posts in PeerTube)