Facebook has the unique ability to track conversions back to users instead of cookies. This means you can track the same user across all devices as long as they're logged into their Facebook account. By comparison, Google Analytics relies on cookies,meaning all the tracking happens exactly on the browser where the cookie was dropped. Even though Google does provide some user-based data; it’s just nowhere near as robust and detailed as Facebook. The result is that when a Facebook user clicks your Facebook ad on their smartphone and visits your site without purchasing, then later returns by typing the web address into their browser on a desktop PC while logged into Facebook, Facebook reports that purchase as a Facebook conversion. Google reports it as a direct source conversion. Again,Google has no idea that Facebook was involved in the conversion.
Facebook tracks indirect “non-linear” conversions, meaning that if a user clicks your Facebook ad and views your site, leaves your site, then returns the next day and makes a purchase, Facebook attributes that conversion to Facebook. Google does not. Google Analytics tracks only direct last-click conversions which attributes 100% of the conversion value to the last channel the customer interacted with before buying or converting — when a user clicks your Facebook ad, views your site, and converts right then and there without leaving.
Facebook conversion reports use a default 7-day window for click-through conversions and a one-day (24-hour) window for view-through conversions. By uniquely tracking view-through conversions, Facebook takes credit for a conversion even if a Facebook user only sees your Facebook ad without clicking it, then visits your website and makes a purchase. Furthermore, Facebook doesn’t differentiate between the two types of conversions; click-throughs and view-throughs are combined into a single data point for total conversions.
View-through conversion tracking is something Google can’t do for Facebook traffic to your site. In contrast, Google Analytics records this conversion type as a direct source conversion. As far as Google is concerned, that same user typed your web address into their browser without influence from another online source.(Tracking parameters like UTM codes can’t help you here.) Google has no idea the user saw your Facebook ad.