For those without Facebook:
However these technologies protect only the direct connections people make to Facebook. People also receive information from us over channels such as email. Whilst Facebook seeks to secure connections to your email provider with TLS, the stored content of those messages may be accessible as plaintext (with attachments) to anyone who accesses your email provider or email account.
To enhance the privacy of this email content, today we are gradually rolling out an experimental new feature that enables people to add OpenPGP public keys to their profile; these keys can be used to "end-to-end" encrypt notification emails sent from Facebook to your preferred email accounts. People may also choose to share OpenPGP keys from their profile, with or without enabling encrypted notifications.
You will be able to update your own public key, using a desktop browser, at:
https://www.facebook.com/me/about?section=contact-info