Employees of Facebook are geeky enough to use GPG. I suspect it was a feature built for internal users, but problematic now as most people don't understand it or are able to use it.
It’s at least a small intersection. I had it set up and I think it’s the only social media company that supported this.
Comically I forgot all about it until I had to reset my password and got an encrypted email. Was a pain to dig out my keys and decrypt it, but it worked.
I used mailvelope for the longest time as a gmail user until I found a better mail app with gpg support. It was pretty slick last time I used it.
Shamefully I also turned on the pgp messages from Facebook. I never found value with it, but to this day it’s still enabled and I don’t care to log in to Facebook to disable it.