Lots of ways this could happen but people often forget about GPS/location services which are very accurate. I'm not sure if you have never had FB app on your ph

Lots of ways this could happen but people often forget about GPS/location services which are very accurate. I'm not sure if you have never had FB app on your phone, but even if you never installed FB app on your phone there could still be geo-associations inferred. Think about the times your phone is near your friends phones, how your phone probably sits in the same space every night, the overlap between your IP location to whatever other devices connect behind your NAT which also send data to FB.