Just FYI: With facebook now (for quite some time) integrating/implementing Jabber/XMPP -- OTR (Off the record messaging) works fine with facebook messages right now, if both users connect to facebook chat with something like Pidgin.

I guess the deniability part is sort of out the window, seeing as how facebook will be logging the traffic pattern(s) -- but you might at least claim that "no, that wasn't what I said".