On Competing with Circles
Dhanji R. Prasanna, former Google+ engineer:
Now, I’m not saying that Circles is the one killer feature to bring down facebook — not at all. What I am saying, however, is that [Google+ and facebook] are not playing on an even field. Like Microsoft and online Office, it is incredibly difficult for facebook to make fundamental changes to their product suite to answer competitive threats. It is for this reason I feel that Google+ has a genuine shot at dethroning facebook.
Facebook’s answer was Groups, but its a tiny hidden feature, and the default way of sharing with Facebook is still to share with your entire friend list.