I'd have to see these Circles in action, but if they're what I think they are (e.g. you'd make a "Work" circle, a "Family" circle, a "Casual Acquaintance" circle, etc.), then Diaspora has that concept: They call them "aspects". I must admit, "circle" seems like a better term than "aspect", though.
I would agree that it's not a very complex or innovative idea. I'm assuming Facebook is intentional in not doing the same sort of thing, aiming for simplicity (everyone in one circle).
