We know it's true, now it's been scientifically proven. (At least pending an independent study for confirmation?) When a book gets onto p2p networks, there is a sales peak, not a decline:
Brian O'Leary, founder of publishing consultancy Magellan Media, measured the impact of peer-to-peer piracy on titles published by the US house O'Reilly for 71 weeks. At today's Tools of Change conference in Frankfurt, the first in Europe, he revealed that while non-pirated books (both print and e-books) showed a "trending decline" after an initial sales peak, the sample titles saw a second peak at the onset of piracy. From week 19, which is on average when titles began to be pirated, to week 23, which was the average second peak, sales rose 90%.
From what little sales I have, I can confirm. Many buyers explicitly mention when buying my book, that they already read it online, they just want to buy it because they liked it.
PS: Seems like having friends is now patented. I'm glad I'm not infringing!