the facebook/messenger app upload all contacts. if he has your phone number, and your facebook account is registered with the same number, you will see a friend

the facebook/messenger app upload all contacts. if he has your phone number, and your facebook account is registered with the same number, you will see a friend suggestion. i don't use any of facebook's app on my phone and a few time's i see a freind suggetion of people i just met in a week or less.