Facebook ad platform Atlas will target users on the Web

SAN FRANCISCO -- Facebook is going to sell ads targeted to its 1.3 billion users when they are elsewhere on the Web in its most direct challenge to Google yet.

​On Monday, Facebook is rolling out an updated version of Atlas that will direct ads to people on websites and mobile apps.

Atlas could create a new revenue source for Facebook and a valuable tool for marketers looking for an alternative to ad networks run by Google and Yahoo.

It could show the most promise on mobile devices where cookies and other tracking devices don't work as well.

Facebook bought Atlas from Microsoft last year.

In a blog post, Erik Johnson, head of Atlas, said it would help marketers "reach real people across devices, platforms and publishers. By doing this, marketers can easily solve the cross-device problem through targeting, serving and measuring across devices. And, Atlas can now connect online campaigns to actual offline sales, ultimately proving the real impact that digital campaigns have in driving incremental reach and new sales."

The Omnicom Group, one of the world's largest advertising companies, is the first to sign up for Atlas.

Such detailed tracking may well raise privacy concerns for Facebook users, already concerned by how targeted ads are on the service.

Those concerns are evident in the growing popularity of Ello, which bills itself as an anti-Facebook with no advertising.