For the most part, social media users (this one, at least) view ads as a necessary evil that help keep their favorite services free. But occasionally, posts for weight loss solutions are replaced with ads for something you are actually compelled to purchase.

Facebook, of course, provides advertisers with stats about how well their ads are performing, but as more and more people shift from the desktop to mobile, the social network said today that it will provide more granular data about how users interact with ads – and make purchases.

“Today we’re launching cross-device reporting for Facebook ads, enabling advertisers to see for the first time how people are moving between devices” before buying something, Facebook said in a blog post.

Did you click an ad on your phone but wait to buy until you were on the desktop? Maybe you perused an advertised product on your laptop, but bought it from your iPad? Facebook will now collect this information and provide it to advertisers so they know where best to serve up their ads.

“With the new cross-device report, advertisers are now able to view the devices on which people see ads and the devices on which [purchases] subsequently occur,” Facebook said.

Facebook has been testing the service with a handful of advertisers, and is now opening it to a wider pool. Those who sign up can see the results on their Facebook Ad Reports: click Edit Columns, and select Cross-Device on the left-hand menu.

As Bloomberg noted(Opens in a new window), the move could prompt concern among privacy advocates who have long been concerned about Facebook’s reach. But Facebook also needs to keep its advertisers happy.

The news comes as Twitter this week rolled out video ads; Facebook started rolling out auto-play ads on the news feed in March. Both companies have also been testing “buy” buttons that appear directly on news feeds and timelines.