Facebook's adverts will now follow users around the web, as the social network goes head-to-head with Google.
Facebook has rolled out an updated version of its advertising platform, giving it the power to sell ads finely targeted to its 1.3 billion users whether they are on a Facebook site or not.
When a user logs into Facebook with their device, it registers a unique device identifier with the company's servers.
The identifier can be used to track a customer so that if another app or website asks for an ad, Facebook can use information about the customer to find the best ad to display.
Omnicom Group, one of the world's largest advertising companies, is the first firm to sign up to the service.
Erik Johnson, head of Atlas, said that cookies - used by the likes of Google to deliver tailored adverts - are flawed and that data provided by Facebook users about themselves is more useful for targeting ads.
In a blogpost he wrote: "People spend more time on more devices than ever before.
"This shift in consumer behaviour has had a profound impact on a consumer's path to purchase, both online and in stores. And today's technology for ad serving and measurement - cookies - are flawed when used alone."
Facebook bought the Atlas advertising platform from Microsoft last year.
The internet advertising industry is worth an estimated $140m (£86m) - and Google still receives the lion's share of ad spending with around 32% market share. By comparison, Facebook sits at just 7.8%.