600k is a sizable book. The Adventures of Huckleberry Finn is 600k. And that's the average per-user; most users will never send or receive messages. The only t

600k is a sizable book. The Adventures of Huckleberry Finn is 600k. And that's the average per-user; most users will never send or receive messages. The only thing I can imagine is that they do an incredible amount of activity tracking.