Meta can already target non-Meta users on third-party sites via shadow profiles, ActivityPub doesn't add magic immunity to that.
In fact fedi is great for targeting. Hashtags posts, favourites, follower/following ... those are gold mines. Even instances can useful. I'm sure with Meta's already working advanced AI techniques to infer that users on a hypothetical Disney instance are good targets for Disney-related material.
Darnell Clayton :verified: (npub1vzc…zat5) Marti Abernathy (npub1j6g…mq7e) Mastodon Migration (npub1kxu…9fca)