Don't know how many will read this now, I'd just like to point out that since you can only message addresses/accounts you're "friends with" on Fb AFAIK, you can effectively broadcast to all your friends without Fb knowing the plain-text recipients.

It is simply necessary to have the plain-text in a publically known format, e.g. prepended with date or name or whatever. Then you post the cypher-text to your "timeline" (or whatever it's called) and try decrypting all your friends' posts to find which one is meant for you.

To get around Fb-imposed usage restrictions use a steganography or obfuscation tool. https://seecret.io is already linked to somewhere on this thread.