Why Facebook Retargeting?
While everyone has probably seen Facebook retargeting, not everyone knows how it works.
Have you ever noticed that after visiting a website, suddenly an ad for that site pops up in your Facebook feed or on other websites or a mobile app? When you visited the website initially, it put a code into your browsing history, allowing it to follow you around the web. When you visit a different site or Facebook, the code triggers those sites to show the initial site’s ads.
Source: Social Media Examiner