Where to Find Your Hidden Messages on Facebook
Facebook identifies messages it thinks are unwanted and redirects them away from your inbox. If someone reaches out to you on Facebook, but he or she is not friends with you on Facebook, or friends with any of your friends, the service has to decide whether that message is spam.
20 Apr 2016 ... If someone reaches out to you on Facebook, but he or she is not friends ... Inspecting your filtered messages is helpful to find out what you have ...