Facebook has added the ability for marketers to track customers across devices when a displayed ad is followed by a purchase, according to a blog post from the company Wednesday. "Cross-device reporting" will allow advertisers to track users' activity across multiple devices, including smartphones, tablets, and computers, to better understand how their ads convert viewers into customers.

Facebook writes that it has long offered "conversion measurement," a metric to show how often a displayed ad results in a successful sale, on a per-device basis. If a user clicked an ad on his or her phone and bought the product it was showing, Facebook would be able to tell that advertiser about it. Now, if a user views an ad on his or her phone and then later gets on a computer to buy the product shown, Facebook will be able to know about those actions and report them back to advertisers, too, all thanks to the ubiquitous Facebook login.

In an infographic accompanying the post, Facebook pointed out that cross-device conversion rates can be as high as 32 percent for a clicked ad within a time span of 28 days. While the new metric does give a new concrete dimension to marketers, it has another benefit: showing how effective Facebook advertising can be on Facebook users.

In terms of privacy, this feature reaches new and creepy heights, though Facebook is not out of bounds given its privacy policy. And it's nothing customers will feel too acutely—it's just that ads will gradually get better, Facebook will grow more powerful, and we will slowly buy more and better-targeted objects.