This news is almost 2 years old now but I just got back onto Facebook a few months ago and found this to be interesting.
Facebook allows you to upload your public key so you have have all of your notifications encrypted.
I haven't fully enable it yet, I've just sent the test email to my account and it worked just fine. I haven't enabled it yet because right now I using Gmail's web interface as my mail "client" as a test (more on that later). And the last time I checked (plus a real quick search right now) doesn't show any sort of GPG integration with Gmail. But I may yet switch after I am done (if I am done) with the Gmail web interface.