According to the Wall Street Journal, Facebook had agreements with "a selected group of companies, some of them had special access to user information." As the publication emphasizes, such agreements operated "well after that moment in 2015, when the social network said that it completely deprived developers of access to this information."
The article notes that the new information its sources provided is corroborated by court documents which the journalists familiarized themselves with.
Facebook, in particular, allowed them to receive information about users' phones, as well as statistics, from which it was possible to determine how close their relationships with each other were.
The publication notes that many of the agreements in question were not related to the agreements that Facebook had with at least 60 major phone manufacturers and makers of other electronic devices.