Big arguments (um, discussions) on several groups I read concerning what's "best" for authors looking to sell their work: Facebook, MySpace, Crimespace, Twitter, etc.
I guess I have it wrong, because I don't necessarily join a group to sell books. Yes, I would like it if people everywhere bought my books, but I enjoy the hour I spend each morning reading comments, learning what others do and don't recommend, and writing my own admittedly idiosyncratic observations. Some sites connect me with people in the business. Others connect me with "real" contacts, usually students I taught who have moved on to other things.
I take no interest in whether or not anyone goes out to buy my book because of something I said online or the mere fact of my presence here. I just enjoy the "socialness" of this sort of networking. If it leads me somewhere, that's nice. If it doesn't, I'll just enjoy sharing joys, gripes, opinions, and pictures.