Any user that sees a Facebook ad must be logged into Facebook, this allows Facebook to track every single ad a user sees. When a customer buys a product on your website Facebook is able to identify that user through their Facebook login in the browser. This allows Facebook to match every ad view or ad click to a conversion event like a checkout.
Facebook can not only track click through conversions but also to accurately track view through conversions. This is a critical competence as customers continue to use a myriad of devices during their purchase decision process. It enables you to attribute events like a user seeing an ad for your product on their phone and then browsing to your website on their PC to buy a product.
Facebook allows you to report attribution on a 0, 1, 7 or 28 days post view or post click basis. Click the Number of days, change to the number you wish to report and click the blue tick button to save.