When people visit your Facebook page, if they click the About link under your icon photo in the header they will be taken to a page that displays various information you have entered about yourself: schools attended, employers, contact information, etc.
When you set up your personal FB page you may have entered an email address to be displayed to friends or a wider audience (depending on your privacy settings), or you may have decided NOT to list any email address.
If you haven’t revisited this choice recently, Facebook has played a little joke on you. They decided to give everyone an email address, yourname@facebook.com… AND they decided to replace whatever other email address you listed as your primary contact information (or no listing at all) with this new Facebook email address!