The social network uses a variety of approaches to collect information about web users. When you load a website for a brand, Facebook pixel sends information about the device and its browsing activities back to the company. The social network can then use that information to help brands target you. Facebook says it does not build profiles on people who are not on the social network, nor does it serve targeted ads to them. Facebook has often defended targeted ads by saying that internet users are annoyed when they see irrelevant ads.
Source: New York Times March 21, 2019 09:02 UTC