https://www.bleepingcomputer.com/news/security/facebook-bug-caused-new-posts-by-14-million-users-to-be-shared-publicly/

Facebook just can't get it together as we learn about another major privacy breach on their platform. This time it was caused by an internal bug that caused any new posts created by 14 million Facebook users to be posted publicly rather than using their default setting.

When posting on Facebook, users have the ability to specify who can see their posts by using a drop down menu where they can select "Public" (anyone), "Friends", or "Friends and Connections".  This drop down menu is called the "audience selector" and will retain the setting that you previously used for new posts going forward.