Facebook gathers information about what you buy — both online and in physical stores — in order to serve you ads that correspond to those purchases.The practice is one of many ways Facebook leverages its wealth of user data to offer advertisers tools to target specific audiences.This is a breakdown of how Facebook learns about your offline activity, and how to opt out.
This is how Facebook learns what you buy at physical stores in order to show you relevant ads — and how to opt out by Aaron Holmes on Business Insider