https://www.cnet.com/news/facebook-unintentionally-uploaded-email-contacts-from-1-5m-users/

A Facebook spokesperson confirmed that 1.5 million people's contacts were collected in this manner since May 2016 to help build Facebook's web of social connections and recommend other users to add as friends.

"Last month we stopped offering email password verification as an option for people verifying their account when signing up for Facebook for the first time," a Facebook spokesperson said. "When we looked into the steps people were going through to verify their accounts we found that in some cases people's email contacts were also unintentionally uploaded to Facebook when they created their account.