FaceBook is NOT a secure site.
It was designed as an open access site with no security at all.
All the security has had to be added on as an extra, the result is that a competent 'Hacker' can get into ANY facebook information in less than 30 seconds.
I have been saying this for years on here , NEVER put anything on FB that you do not want the entire world to know, there is no way to keep it secure despite what FB says or what settings you use. I have never used FB and never will.