The following comes from the June, 2011 issue of Consumer Reports:

If you’re like some 150 million Americans, you share the details of your life on Facebook, assuming that you and other users are its main customers and that it’s accountable to you. But Bruce Schneier, chief security technology officer at security firm BT Global Service, says you’re not Facebook’s customer. “You are Facebook’s product that they sell to their customers,” he says, referring to the network’s advertisers.

But don’t just single out Facebook for this observation. This is true of all ad-supported media.