How can Google, Facebook, etc find out which website you came from - when there is no referrer?

Question

Let's say Site A is a traffic generating website. User A clicks on a banner on Site A and gets redirected to Site B (for example a landing page). From Site B there is a link to Site C (which is for example Facebook, Google, or any other big company).

Question: How can Site C determine which sources did User A come from? Becuase I have seen a lot of Site C websites that knew which was Site A .

As far as I know, browsers cannot pass Site A referrer url to Site C . How do they do it anyway? Are they using any kind of PHP or Javascript code for that?

Answer 1

The way they know where you have been is because they drop cookies with unique IDs on your computer so they can track you.

For example if a website uses any Facebook widgets, Facebook is already tracking you and knows what ads to target you with based on the sites you have visited once you go back to Facebook.com, or if Google Analytics is on the website (which it probably will be) this is tracking everything you go on from one site to the next and doing the same with serving relevant ads.