Facebook knows who your friends are. Google knows what you’re interested in finding on the internet. Amazon knows what you’ve bought � has a pretty good idea of what you might want to buy. If you were an advertiser, which company’s data sounds most valuable to you?

But ads have the potential to do even more for Amazon’s bottom line. The company’s sites are viewed by more than 100 million different people in the U.S. every month, which ranks Amazon sixth behind just Google, Microsoft, Yahoo, Facebook, and AOL on the list of most-visited sites, according to comScore. The top five companies' web business models depend above all on advertising. Amazon’s doesn’t. But with traffic that ranks them up among the web’s most valuable media properties, comScore analyst Andrew Lipsman says Amazon has no reason not to jump into the ad game.