Facebook users can add PGP keys to their profiles in order to receive end-to-end encrypted notification emails sent from Facebook to their preferred email accounts. Using PGP technology to send emails keeps potentially sensitive messages out of the hands of hackers and other snoopers. This is known as one of the most popular email encryption standards that uses public key cryptography and Facebook has chosen to use GNU Privacy Guard GPG a widely used and free implementation of the OpenPGP standard. Facebooks move will bring encryption to the fore, but most people have no idea how public-key cryptography works.”]
Source: http://securityaffairs.co/wordpress/37425/social-networks/facebook-use-pgp.html