The ‘experimental’ feature allows end-to-end encrypted notification emails from Facebook to your email accounts. You can list your OpenPGP key on your profile whether or not you choose to switch on the encrypted messages feature.
If encrypted notifications are enabled, the company will sign outbound messages with its own key.
In order to manage your PGP key, you’ll have to use a desktop, but Facebook is looking into public key management on mobile devices for the future.
➤ Securing Email Communications from Facebook [Facebook]