Bruce Schneier reports on research that geotags Twitter users by mining their social graphs.
The method seems pretty successful:
Leave-many-out evaluation shows that our method is able to infer location for 101,846,236 Twitter users at a median error of 6.38 km, allowing us to geotag over 80% of public tweets.
Lots of tweets were accurately tagged to within 1km.
It’s getting harder to not reveal private information about our friends by accident.