Take this all with a grain of salt, but supposedly Zuckerberg responded to an IM regarding his users and why they would trust him. He allegedly said:

Zuck: I don’t know why.Zuck: They “trust me”Zuck: Dumb fucks.

Facebook has since issued this statement on the matter:

The privacy and security of our users’ information is of paramount importance to us. We’re not going to debate claims from anonymous sources or dated allegations that attempt to characterize Mark’s and Facebook’s views towards privacy.Everyone within the company understands our success is inextricably linked with people’s trust in the company and the service we provide. We are grateful people continue to place their trust in us. We strive to earn that trust by trying to be open and direct about the evolution of the service and sharing information on how the 400 million people on the service can use the available settings to control where their information appears.