The story should be identified along with anyone who saw it – and they do record that because its part of the sales algorithm, and users who saw it should find a retraction and correction top of their feed next time they log in.
That will un-do the damage caused by those story’s and solve this shitty situation. The answer is so simple that the reason FB doesn’t can only be that they don’t want to. It would make those ads largely pointless cutting off much of their revenue stream.
This can, and should be implemented retro-actively. A lot of people today believe a lit of stupid things because they read them on FB. Older posts (say 5 years back) which were flagged and then confirmed by fact-checkers to be BS could be condensed into a single ‘all the times you’ve been lied to’ post. The world would become a better place within a week!