I used to play a phone game and utilized mock locations for area specific content. Later, Facebook would suggest friends from the specific location I had spoofed, despite having never once set foot there or having ever known anyone from the area.
Myself and someone I just met were suggested as friends to each other after we met. Neither looked up the other. Neither created any contacts in our phone. Only possibility is our FB app knew we were both in the same place at the same time for a few hours.