Google+ may not have killed Facebook, but it is developing into a nice place for tearoom style chats about math; less formal than MathOverflow, more characters than FB. This thread Allen Knutson started about circle packing is a case in point. If I’m reading the thread and I say to myself “Matt Kahle should be weighing in on this,” I can just type in his name with a + prepended to it — and he’s summoned! That’s a functionality that really doesn’t exist elsewhere.
