Great line from Fast Company's interview with Mark Zuckerberg, "Exclusive Interview: Facebook's Mark Zuckerberg on the Value of Viral Loops":
Penenberg: Marketing to one's social graph offers a number of huge advantages. Why is that?
Zuckerberg: I think the basic idea here is that there is a phenomenon in peoples' interaction. The message that you get, in a lot of ways, is actually less important than who you get it from. If you get it from someone that you trust a lot more then you'll really listen to it. Whereas if you get it from someone you don't trust you might actually believe the opposite of what they said because you don't trust them. I think that's the basis of the value that people get on the site. I go to someone's profile and see that they like this band. That means more to me than if I just saw a billboard for that band. We figured that in the really organic way to make money and sustain the company, that these interests would be aligned.