Joining a public WiFi made the owner able to send me a Facebook friend request. How? I walked by a bar with free, unsecured WiFi. My device auto-joined it, and a minute later I had a friend request on Facebook from that very bar. 

How did they get my Facebook identity just from joining their WiFi? It feels a bit intrusive for them to do that, and honestly it makes me a bit paranoid about how much information about me is made available to the owner of the bar so easily.
Not enough rep for comment, sadly - but my guess is that this could be Facebook beacon (not necessarily BT, they work with WiFi too).