G+ meets almost all of your criteria, only thing it's missing is the ability to toggle discussions public or private after the fact, but you can at least control that aspect when creating the conversation.
I once tried 'chatting' with my girlfriend on google+ using a private channel. The fear of some error happening, and our messages being accidentally shown to public was too much, and we switched back to chat. Google+ us social and public. Not technically. But that is what they are trying to be, and advertising to be.
Though I think gPlus + Wave would be the ideal such environment for me.