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.

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.

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.

Just a small quibble, if you're typing in the textarea the website or any other extensions you have in your browser could spy on the plaintext as it is being typed. A secure browser extension would have to call an external program, let you type and encrypt your message in there, and then deposit the encrypted message back into the textarea.

Wouldn't you have to encrypt the text repeatedly, once for each recipient?

gpg handles this natively.

http://www.gnupg.org/gph/en/manual.html#AEN111

"multiparty encryption" is what you're looking for.