There is this old concept on facebook called "tagging faces". Someone who taken pictures of you has tagged it allready, mostly not just one person so fb probably has pretty good biometrics of your and my face. They scrape contact lists as well and other grey patterns to obtain your contact info. It is depressing how for each privacy activist there's dozens of useful idiots who will tag you and publish your phone number on fb in spite of you not wanting them to feed surveillance systems.