GPG/PGP keys were the greatest missed opportunity for social networks. For example widespread acceptance of gpg/pgp would have been a great way to reduce spam.
About ten years ago I had two customers who liked us to all encrypt all email. It was easy to do and fairly transparent - strange that this never caught on, at least in my experience.
The best way to push the adoption of GPG is through a HTML5 crypto API that would include it. A social network or email service that doesn't work in the browser would be inaccessible from cyber-cafés or from work (two of the most popular accesses to the web in developing countries).
I usually try to avoid sweeping generalisations, but in this case... I just can't see any use at all for crypto communication where the end-points are not secure.
