It has come to my attention that the default Facebook privacy settings are not properly functioning at seemingly random times. Unfortunately you have no idea the settings aren't working correctly until after you post something...
Despite my default setting of "Friends Only", I've noticed multiple posts being available to everyone, as indicated by the globe icon below.
It may be worth checking your old posts to see who has access to them. You may be surprised to find some of your posts were set to public.
I have posted about this issue on Facebook's support page, but expect little response.
One possible workaround to this issue is to go into your Facebook Privacy Settings, change your post privacy to public, then back to Friends Only. This appears to have corrected the issue for me, for now.
If anyone else has encountered this issue, I encourage you to post a comment, and if you managed to fix it or not.