I suspect FB uses location and/or WiFi data as one of entry points. Note that smartphones often scan WiFi networks even when WiFi is off. Based on the high int

I suspect FB uses location and/or WiFi data as one of entry points. Note that smartphones often scan WiFi networks even when WiFi is off. Based on the high intersection of two sets of SSIDs names visible from two devices at the same time, you can decide two people were in one room together. If this happens regularly, you can be quite confident people "know" each other in some way, which can unveil many surprising "friends" recommendations.