An update rolled out today by Facebook allows users to post their public email encryption key on their Facebook profile, so others can encrypt future emails to that user.
Here's the official blog post at Facebook.
Facebook profiles now have a field for PGP public keys–just like for phone numbers or email addresses. Uploaded keys can be shared as widely or narrowly as desired, just like other information on a Facebook profile. For journalists who use Facebook to connect with sources and disseminate, share, and comment on news, their profile will now indicate they are available for encrypted emails. The new feature will also make it easier to securely contact potential sources.
"Status update: Facebook users now have access to PGP encryption" and "CPJ welcomes Facebook move to add PGP encryption features" [Committee to Protect Journalists]
"Securing Email Communications from Facebook" [Facebook]