You do not understand correctly.

Facebook (and other ad networks) want credit if someone clicks on your ad, spends time on your site, but then doesn't actually buy anything until a couple days when they enter the URL directly (i.e. not clicking on an ad). This is the problem generally referred to as attribution. Facebook (and others) do this by tracking everyone all around the web. They track the ad click and then they track you going back to the website the next day, and they know it's the same person.

Ad attribution directly depends on pervasive tracking.