Around 14 million people have default sharing settings set to “Public”” for any new Facebook posts created. This means any posts created by these users could be read by anyone regardless of their default setting. Facebook engineers spent an additional 5 days resetting any new posts made by those affected to the default setting they had previously been using. Facebook will be displaying privacy notifications that appear when a user logs into Facebook via the web or mobile app. These notifications will be used going forward to report privacy issues or breaches.”
Source: https://www.bleepingcomputer.com/news/security/facebook-bug-caused-new-posts-by-14-million-users-to-be-shared-publicly/