To serve you relevant ads, Meta will use your activity on Threads and Instagram, the posts you interact with, your email address, and “your activity from off Meta technologies,” according to a support page.
Do I read it right in that they'll try to follow you around Fediverse, aside from monitoring you on every site they already have a nose on?
Probably more about their general user tracking than that tbh. Meta has always had profiles for everyone because of Facebook/meta integrations on every website
Exactly, all info on lemmy is public, defederation would help neither with 3rd party cookie tracking (the real situation) nor with tracking on other instances (what the user imagined they are doing).