It would be difficult to connect you to a Meta account to serve ads to because they only have your user name, profile pic, server IP, and server domain name. In most cases it'd be impossible. You're pretty well protected because Mastodon servers treat all remote servers as untrustworthy and don't give them any info.

Facebook already creates "shadow profiles" for people not on Facebook and stores data about them. This means Meta won't directly monetize the fediverse, but use the data available for their ad business anyway. (Maybe even connect other accounts through posts, but I don't know how well this works with the info and amount of a users posts.)

Nothing stopping them from doing it now, anyone posting to the fediverse has to accept that their posts can and probably will be used to train someone elses LLM. It's public afterall.

[1] https://www.howtogeek.com/768652/what-are-facebook-shadow-profiles-and-should-you-be-worried/