[wonderzombie]

Without meaning to detract from some of the excellent explanations in this thread, I should add that it sounds like you're approaching this as if it were mostly a technical problem when as far as I can tell it is mostly a social problem. Social problems are harder than coming up with a tool to make it understandable, although I strongly suspect that's also not as easy as you think it is. The failure modes, corner cases, and generally making it robust to someone who Just Doesn't Get it are what kill you, not the happy path of a user who does roughly what they ought.

Setting aside the social problems of explaining cryptography and generating a network effect, most people assume their communications are not worth listening to. "If they want to listen to/read my mom drone on, they're welcome to it!" Or they assume it happens to "someone else."

I'd expect that most people -- rationally and correctly, I might add -- conclude that it's unlikely to happen to them in any way relevant to their experience.

Anything you suggest has to overcome that inertia. Facebook was a value-add. Cryptography's value-add is subjectively nil and possibly negative (whoops laptop stolen lost my keys) if you don't see the benefit in the first place.