From Phil Baker at PJ Media:
When people sign up for Facebook, they’re encouraged to upload their contacts to make it easier for Facebook to connect them with their friends. That allows Facebook to access personal contact information for people who never signed up for the platform or gave their permission to share their information. Facebook knows that these contacts are friends of the new Facebook user, and can start compiling additional details on these non-members.More.
Reality check: Facebook’s business is farming you. Eventually, it will not matter if you are on Facebook or not.
See also: Wait till the campus Cools are on the other side of the admin desk