You might have another app using the FB SDK (as described in the app). Or .. they have a database from mobile IP to rough location - or even, without knowing y

You might have another app using the FB SDK (as described in the app). Or .. they have a database from mobile IP to rough location - or even, without knowing your exact location, perhaps both of you were using the same convention free WiFi, or the same Verizon proxy (or whatever), which would indicate close proximity even without giving the location directly. Are you using iphone or android?