From the Facebook “Secure the Graph” Page
It’s very important to us that the people who use Facebook feel safe and can trust that their connection to Facebook is secure; for instance this is why we run connections to our site over HTTPS with HSTS and why we provide a Tor onion site [https://facebookcorewwwi.onion] for people who want to enjoy security guarantees beyond those offered by HTTPS.
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.