LAST week, Facebook filed documents with the government that will allow it to sell shares of stock to the public. It is estimated to be worth at least $75 billion. But unlike other big-ticket corporations, it doesn’t have an inventory of widgets or gadgets, cars or phones. Facebook’s inventory consists of personal data — yours and mine.

Facebook makes money by selling ad space to companies that want to reach us. Advertisers choose key words or details — like relationship status, location, activities, favorite books and employment — and then Facebook runs the ads for the targeted subset of its 845 million users. If you indicate that you like cupcakes, live in a certain neighborhood and have invited friends over, expect an ad from a nearby bakery to appear on your page. The magnitude of online information Facebook has available about each of us for targeted marketing is stunning. In Europe, laws give people the right to know what data companies have about them, but that is not the case in the United States.

Facebook made $3.2 billion in advertising revenue last year, 85 percent of its total revenue. Yet Facebook’s inventory of data and its revenue from advertising are small potatoes compared to some others. Google took in more than 10 times as much, with an estimated $36.5 billion in advertising revenue in 2011, by analyzing what people sent over Gmail and what they searched on the Web, and then using that data to sell ads. Hundreds of other companies have also staked claims on people’s online data by depositing software called cookies or other tracking mechanisms on people’s computers and in their browsers. If you’ve mentioned anxiety in an e-mail, done a Google search for “stress” or started using an online medical diary that lets you monitor your mood, expect ads for medications and services to treat your anxiety.

Image Credit... Joon Mo Kang

Ads that pop up on your screen might seem useful, or at worst, a nuisance. But they are much more than that. The bits and bytes about your life can easily be used against you. Whether you can obtain a job, credit or insurance can be based on your digital doppelgänger — and you may never know why you’ve been turned down.