The information being transmitted is one of Facebook’s basic building blocks: the unique “Facebook ID” number assigned to every user on the site. Since a Facebook user ID is a public part of any Facebook profile, anyone can use an ID number to look up a person’s name, using a standard Web browser, even if that person has set all of his or her Facebook information to be private. For other users, the Facebook ID reveals information they have set to share with “everyone,” including age, residence, occupation and photos.
The apps reviewed by the Journal were sending Facebook ID numbers to at least 25 advertising and data firms, several of which build profiles of Internet users by tracking their online activities.