interesting concept, but from what I gather, sites would have to fully cooperate; they're rather sketchy on the details of the whole thing, but the whole "shared-key" thing doesn't really work so well unless the site itself promises to never look at the decoded data; either that, or it becomes browser-side and thus needs javascript.
And of course if the information is posted elsewhere, or the data from the facebook (or other participating page) is copied or, more likely, archived, that archived data will be the decoded text (unless, again, they use a client-side solution of scripts) so I imagine there would be some kinks to work out. Personally I sort of question the necessity of such a thing, but I don't use facebook or twitter or any of those sites, so maybe I'm just not hip enough to get it