It looks like Facebook it up to its usual tricks by making changes to your profile without your consent. This time it is your email address.
Remember eons ago Facebook tried to convince everyone that we should use FB messenger to communicate and we all had a new email address from them. Well FB have decided to promote this email address.
If you choose to show your email address as part of your Facebook profile it may have disappeared and been replaced by a facebook.com email. Click on your Name in Facebook, then ‘About’ in your Facebook Timeline. Look in the ‘Contact Info’ section. See a facebook.com email address? Probably.
You can of course change it back to show your own email, or both!
I only show my email address to friends on Facebook; how dare they change it to their own service. What makes them think I want emails coming in to be routed through Facebook. Apparently the emails are simply forwarded on to your real email address but I have personally yet to see this work, and emails to my facebook.com address just disappear. Great!!