There is nothing at all novel about the peculiar, ambient intimacy of social media if you have spent any time living in New York City. Life in Manhattan is like living inside a gigantic Twitter stream. What you get to know about people you don't know simply by accidental adjacency is astonishing. For a few years, a guy who lived in the building across the street from me practiced piano every day in the nude. He had double-height windows in his apartment and had positioned the piano to take...