Presumably if Facebook really wanted to track you they could associate requests from the same IP and request signature (user-agent/accepts etc)
I would bet $100 FB does geolocate your IP to determine if it's likely a single residence or a larger multi-user building, and likely tracks that, but that misses a large chunk of their user base.
Imagine the number of people who use FB as a distraction at work or in class, but might not use it as much at home.