I think this is far less mysterious than people suppose.<p>Hypothesis - if someone on facebook views your profile then facebook is more likely to suggest them as a friend.  Increase the probability if the person is a low degree of separation from you.<p>Obviously people on dating apps are often going to be searching each other out on facebook to see more info.<p>I guessed this was how facebook did it because I saw an ex of mine once on the street (I don&#x27;t have the fb app on my phone or anything like that - so I doubt it was using location data).  We never spoke - but made eye contact.  Later that day she appeared as a friend suggestion for the first time.  My guess is that she viewed my profile out of curiosity.