Facebook shares the data which you share with them for the purpose of 'security reasons' with advertisers. Facebook seems to be surrounded by a lot of controversies these days.
Facebook has recently introduced a new feature which is known as Custom Audiences that works on Personally Identifiable Information (PII) of Users. In this, Advertisers are allowed to target specific users.
What sources of PII are actually being used by Facebook?
Facebook allows its users to share contact information
on their profiles. These numbers when added are not displayed to other users unless they are verified. This becomes one of the most direct and explicit way of providing PII to advertisers.
What Control do Users have over PII?
You have the liberty given by Facebook in order to choose who will be able to see PII listed on their profiles, also the current list of possible general settings being either Public, Friends or Only Me. Also users will be allowed to restrict a specific set of users in order to enable them to search for them using their email address or their phone number.
What Information about How Facebook uses PII gets disclosed to the users?
When you add mobile phone numbers
directly on your Facebook's profile, no information regarding the uses of that number is disclosed to them. The number gets disclosed only to those users while they add a number from the Facebook website.