[reeses]

This is a solved problem, including the problem that you're haven't anticipated, which is the helpful "autosave" of drafts.

http://www.emacswiki.org/emacs/Edit_with_Emacs

I'm sure something similar exists for vi, sublime, etc., but emacs (of course) has great gpg support (http://www.emacswiki.org/emacs/EasyPG).

This moves the burden to your local machine, which, while not guaranteeing privacy, helps reduce the amount of data that you're just handing to the bastards.

[mverwijs]

Hardly solved. I want it to be so simple the average Facebook user will want to use it.

Emacs hardly fits that bill. Nor does popping up any other editor I think.

I would just like to use the native browser interface and right before doing a POST, have the browser do a pop up and ask me which of my friends I would like to share the text with.

[mverwijs]

Also, there is the decription part. The browser would need to auto-decrypt on the fly all the encrypted messages that are meant for your eyes only. Without asking for a password every time. And without having to open a new application. Rendering pages might prove difficult, as the page can only be rendered properly after decription.