Not enough info to be sure, but here's my guess -- you connected to Facebook from the Oxford Tube so I'm assuming you mean you were using the wifi at this station? FB could tell via geolocation where you were connecting from, and once it knew you were there, probably it just checked its database and saw that people who connect from that location at that time who have already told FB where they are from, are most often from those places. And it was probably accurate either just because of a lucky guess and/or train arrival patterns. So at a guess, it's not actually as invasive/creepy as it seems, just looks that way thanks to the magic of big data. :P