You have value--and not just as a good friend, loving family member, and upstanding member of society. You're also a valuable commodity that companies buy and sell. Your age, browsing habits, and friends lists are all hot properties. And yes, all this data is recorded, packaged, and sold to the highest bidder by your favorite websites.
This shouldn't come as a surprise, of course. Advertising is the lifeblood of the Web, and without it we'd all have to pay for content that currently comes to us free. But make no mistake: Every time you surf a modern, advertising-supported website, you relinquish personal information to the site's advertising partners, which use this data to target particular ads to particular people.