Research published in a recent edition of Science suggests that analyzing patterns of cellphone use which includes locational data could be upwards of 93% successful in predicting the behavior patterns of users.
Combining this with sorely lacking privacy features that most social network users fail to toggle "on" in their account settings which allow datamining services like RapLeaf to compile, the ease of IP Address identification and new techniques for tracking people's locations based upon email trace routes being developed by Google, I am sure that astute weilders of information tools could get this certainty up by a few more percentage points.
Heck, enable your pda, notebook, tablet computer or cellphone with GPS and I can locate you 100% of the time, minus when you forget the phone in a bathroom or leave it at work.