Saw a mobile ad, bought on desktop? Facebook now follows that

Facebook is now able to track how ads impact consumer spending across multiple devices, the company said Wednesday. The move is potentially significant as people increasingly use Facebook on many different devices, and as marketers look for new ways to measure the effectiveness of their campaigns.

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Tracking user behavior on mobile devices has long been a challenge for marketers because of the lack of "cookies" which enable consumer tracking on the Web. But Facebook is the first company to be able to connect the dots between what ads people see on one device and what purchases they make on another.

Here's how it works: Marketers will be able to view the number of consumers that clicked on an ad in their Facebook news feed on their iPhone, and then subsequently made a related purchase on the desktop, or, conversely, saw an ad on the desktop but then purchased on a tablet or phone.

The number of consumers who shop this way is potentially huge, says Altimeter Group. The research firm recently found that more than 60 percent of U.S. adults who are online use at least two devices daily, and more than 40 percent of them sometimes start an activity like shopping on one device only to finish it on another. (Here's Altimeter's white paper on the topic.)

Facebook says early tests have been a success. "In early tests, marketers have used Facebook cross device reports to better understand the role of mobile in their campaigns and explain the differences in reports by third-party analytics tools that do not account for cross device conversions," the company said in a blog post.

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All that behavioral information is potentially very significant, said Altimeter analyst Rebecca Lieb. "Tracking from device to device has always been complex," she said.

Because Facebook's various platforms—mobile, desktop and tablet—see a lot of use, "they're better able to connect the dots," Lieb said.

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"They're not quite as fragmented as other properties that can't close the technological circle, including Google, Microsoft, Yahoo and AOL," she said.

Lieb says this is a potential game changer for Facebook as it looks to drive mobile revenue to keep up with consumers' shift on to mobile devices: "Advertisers need to know where their advertising dollars are making an impact and where they're not. The IAB released new numbers today indicating that mobile spend has jumped 120 percent worldwide in just the last year. If advertisers are shifting that many dollars they're going to want to know if their dollars are making a difference."

—By CNBC's Julia Boorstin. Follow @JBoorstin