Google will start selling ads on Facebook. The search giant announced on Friday that advertisers who buy ads across the Web through its 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 competitors in the market of targeted ads.

DoubleClick Bid Manager (formerly Invite Media) has been helping agencies and advertisers buy ad space on sites across the web for years.

Facebook Exchange lets advertisers reach users based on their browsing history elsewhere on the Web. Cookies track users' movements so an advertiser can show ads to them based on their surfing habits and interests no matter which web site they follow.

"We're always looking at ways to serve our clients even better - starting in a few months, clients will be able to buy inventory on FBX via DoubleClick Bid Manager," said DoubleClick senior product manager Payam Shodjai.