Facebook’s “Search” tool allows anyone to look up users by entering phone numbers or email addresses into Facebook’s search tool. Hackers then used automated computer programs to feed the email addresses and phone numbers into the “search” box. This scan allowed them to find out the full names of people associated with the names of those associated with their email addresses or phone numbers, along with the Facebook profile information they chose to make public. This information was then more likely to be used by cybercriminals to target particular individual using social engineering or other cyber attacks.
Source: https://thehackernews.com/2018/04/facebook-data-privacy.html