A New Metaphor for Social Networking: "Places"

It does have some similarities to a chat room, yes. I would imagine it to be more persistent and have more multimedia functionality, though. Maybe threaded commenting.

But you do hit on something important, though: the chat room was very easy to immediately understand because there was the metaphor of the room. It was simple to understand that when you talk, everyone in the room can hear you and no one outside the room can.

I'm not sure what you mean by "sharing of circles," but if it allows a similar immediate understanding of information flow, then I'll be happy.