It seems to me that what’s new about Circles (and Twitter’s “Follows” structure) is the weird way they mix the social and the public.
Google Circles are unlike a bunch of people sitting around in a circle talking about stuff, because G Circles are asymmetric: That I’m in your Circle does not mean that you’re in mine. So, when I post to my Circle, it has elements of the social (symmetric communication, the possibility of back-and-forth conversation, and the implication of a continuing relationship) but it also has elements of the public (asymmetric communication, more difficulty engaging in a back-and-forth because of scaling issues, and no implication of a continuing relation).
What are prior analogues of this weird intermingling of the social and the public? We could always be social, and we could always be public (to one degree or another). The casual and often unnoticed mingling of the two seems to me to be genuinely new.
(This expands on my comment to Robert Paterson’s post at Google Plus.)