[grovulent]

I think this is far less mysterious than people suppose.

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.

Obviously people on dating apps are often going to be searching each other out on facebook to see more info.

I guessed this was how facebook did it because I saw an ex of mine once on the street (I don'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.