What Facebook knows about you

Peekaboo.

"Apps" are stylish, discrete chunks of software that live online or in your smartphone. To "buy" an app, all you have to do is click a button. Sometimes they cost a few dollars, but many apps are free, at least in monetary terms, Julia Angwin and Jeremy Singer-Vine write in The Wall Street Journal

You often pay in another way. Apps are gateways, and when you buy an app, there is a strong chance that you are supplying its developers with one of the most coveted commodities in today's economy: personal data.

Some of the most widely used apps on Facebook—the games, quizzes and sharing services that define the social-networking site and give it such appeal—are gathering volumes of personal information.

A Wall Street Journal examination of 100 of the most popular Facebook apps found that some seek the email addresses, current location and sexual preference, among other details, not only of app users but also of their Facebook friends. 

One Yahoo service powered by Facebook requests access to a person's religious and political leanings as a condition for using it. The popular Skype service for making online phone calls seeks the Facebook photos and birthdays of its users and their friends.

This appetite for personal data reflects a fundamental truth about Facebook and, by extension, the Internet economy as a whole: Facebook provides a free service that users pay for, in effect, by providing details about their lives, friendships, interests and activities. Facebook, in turn, uses that trove of information to attract advertisers, app makers and other business opportunities.

Capitalizing on personal data is a lucrative enterprise. Facebook is in the midst of planning for an initial public offering of its stock in May that could value the young company at more than $100 billion on the Nasdaq Stock Market.