https://www.techradar.com/news/millions-of-facebook-users-have-accidentally-made-their-private-posts-completely-public

Facebook is warning users to check their recent posts and photo uploads after discovering a bug in its composer that set the default privacy option to 'public'. 

Usually it's safe to assume that Facebook will use your preferred privacy settings, but unless you're in the habit of checking every time, you may have inadvertently revealed far more than you intended.