Photograph by Adam Berry — Getty Images
By David Meyer
May 26, 2016

Facebook (fb) will close down FBX, the ad exchange that let ad tech companies such as Criteo (crto) and AppNexus buy space in front of Facebook’s desktop users, by November.

The company said on Wednesday that it was instead focusing on its “dynamic ads,” which is where vendors can automatically show ads on Facebook based on the browsing actions of users visiting their own sites, and “custom audience” products, where Facebook matches companies’ mailing lists to its users.

Part of this decision could be about Facebook having more control over its ad ecosystem and the user data in it. FBX didn’t give the company much personal data compared to the other systems, but the move is also about the trend towards smaller screens.

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Crucially, dynamic ads and custom audiences work for mobile as well as the desktop. FBX—a rival to Google’s (goog) ad exchange—doesn’t. Mobile, according to Facebook’s latest quarterly results, now represents 82% of its total ad revenue.

“Mobile is now a necessary component of effective marketing campaigns, and Facebook is helping millions of businesses understand their customers’ purchase path across devices,” Matt Idema, Facebook’s vice president of monetization product marketing, told the Wall Street Journal. “Dynamic ads and custom audiences have mobile at their core and are delivering excellent results for businesses, so Facebook Exchange spending has shifted towards those solutions.”

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As the Journal notes, Facebook has been lowering the numbers of ad tech companies that could buy ad space inside FBX on behalf of advertisers.

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