The War between Apple, Facebook, Google and Amazon

It used to be that Amazon sold books, Google was a search engine, Facebook was a social network and Apple sold computers. Now these technology giants are branching out into all kinds of products and disrupting business across the economy. Farhad Manjoo is a technology columnist for Slate magazine and the author of True Enough: Learning to Live in a Post-Fact Society. He also contributes regularly to National Public Radio, Wired News and Fast Company, for which he recently wrote "The Great Tech War Of 2012" about the battle for the future of the innovation economy. The piece outlines how these companies are competing with each other - and other companies - in markets for mobile phones, apps, social networking, retailing, advertising, finance and much more. Part of the National Affairs Series: The Nation in Transition.