Newsweek's Daniel Lyons may have just hit the nail on the head:
The real holy grail is your list of friends. With that information, marketers can start sending more targeted messages...
The genius of Google, Facebook, and others is that they've created services that are so useful or entertaining that people will give up some privacy in order to use them. Now the trick is to get people to give up more—in effect, to keep raising the price of the service.
These companies will never stop trying to chip away at our information. Their entire business model is based on the notion of "monetizing" our privacy. To succeed they must slowly change the notion of privacy itself—the "social norm," as Facebook puts it—so that what we're giving up doesn't seem so valuable. Then they must gain our trust. Thus each new erosion of privacy comes delivered, paradoxically, with rhetoric about how Company X really cares about privacy. I'm not sure whether Orwell would be appalled or impressed. And who knew Big Brother would be not a big government agency, but a bunch of kids in Silicon Valley?