Up until recently, how a hotel acquired a booking, was sometimes a little blurry, especially when it came to mobile devices.

Today, that is changing. Facebook recently unveiled its Atlas relaunch, which the company acquired from Microsoft in 2013. At the time of its acquisition, Atlas was used by advertisers and agencies for tracking display ad and efficacy. However, this did not take into account mobile devices, with has become the preferred method of the millennial when booking hotel rooms.

The newly unveiled Atlas measures cross-device and cross-platform to leverage targeting capabilities powered by Facebook ID.

This essentially means that Atlas now allows hoteliers to measure their ad campaigns across screens by solving the cookie problem and target potential guests not just across the web — but with mobile devices.

What’s wrong with cookies? They just aren’t working anymore. According to executives on a recent Atlas launch panel at Advertising Week, on average, cookies have a 59 percent tracking success rate, and they overstate frequency by 41 percent. But of particular interest to hoteliers marketing to millennials, cookies cannot measure mobile user activity. With Atlas, though, mobile conversation can be tracked.

In addition, Atlas can use Facebook’s Audience to access Facebook’s targeting precision across the entire web, including mobile. For example, hoteliers could leverage Facebook targeting to reach a potential guest on their website, and then use Facebook’s Audience Network to reach that same consumer on the hotel’s branded app on their mobile device.

For any hotel brand looking to target the millennial guests more efficiently, and thus understand where each booking comes from, this is an extremely valuable new tool.