It seems like the best thing to do would be penalize a site if it shows verifiably false content until it both updates the original story and issues a retraction. Facebook could prioritize retractions in its algorithm and ensure that people who saw the original story would see the retraction as well.

Content providers who abide by journalistic guidelines could be prioritized. Providers who actually have an editor could even associate the editor of each article with the post so that the editor's individual track record could be checked alongside the site itself, which would work better for large organizations who have multiple departments and don't deserve to be globally penalized for a single bad editor.