Google headquarters

A part of Google's Mountain View, Calif., campus in 2007. Google on Friday announced an advertising warm-up with Facebook. (Jeff Chiu / Associated Press)

SAN FRANCISCO -- Google can now sell ads on Facebook.

The search giant announced on Friday that advertisers who buy ads across the Web through Google's DoubleClick service will soon be able to buy ads on Facebook's real-time ad bidding service FBX.

The move signals a rare moment of cooperation between the two fierce competitors. DoubleClick allows advertisers to buy ads on dozens of ad exchanges. But when Facebook launched FBX in June 2012, it did not invite DoubleClick to take part. Facebook has since become a major player in retargeted ads.

Facebook Exchange lets advertisers reach users based on their browsing history elsewhere on the Web. Cookies track users' movements so that when someone checks out a leather handbag on a website but doesn’t complete the purchase, that advertiser can show ads for the handbags on other websites the person visits in an effort to get the consumer to click "buy."

Ads sold this way are still a relatively small percentage of the overall U.S. online advertising market, but demand is increasing. And Google powers a huge amount of this type of ad on the Web.

Facebook clearly did not initially want to add to Google's vast trove of data. It's unclear what warmed up relations between the two tech giants.

"Partnership has been key to Google's success as a rising tide lifts all boats," Google said in a blog post.

Jeffrey Chester, executive director of the Center for Digital Democracy, says the move raises privacy concerns.

"Facebook's ad exchange is a powerful targeting system that melds its own storehouse of user information with data from other data brokers. Google has now joined that private club. With Google and Facebook pooling their vast data profiles, and access to our mobile phones, consumers will have no place to hide," Chester said.

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