From my extremely limited experience of tweeting and facebook updating, I think that the difference is one of quantity versus quality. You can tweet anything any time in any situation. There is not really any sense of etiquette being breached by a large number of mundane posts.
A facebook update however should be more carefully crafted and should have some value for the wider community in terms of wit or wisdom. Etiquette for facebook limits the number of status updates remembering that, in contrast with twitter, people have other things to do on facebook apart from reading your updates.