Ask HN: How does FB's friend suggestion mechanism work?

A recent Reddit thread [1] suggests that FB is able to track location even without the installation of any of their apps. Is this via tracking pixel or through a deal with Samsung/Google?

[1] https://www.reddit.com/r/privacy/comments/9z1205/facebook_suggested_that_i_friend_a_person_that_i/?st=JORDJ5U5&sh=ef5fa5aa