An MIT project reached the same conclusion a couple years ago http://www.independent.co.uk/life-style/gadgets-and-tech/mit...

Someone from MIT also contributed to this study in the same vein http://www.nature.com/articles/srep01376 From the abstract, "[I]n a dataset where the location of an individual is specified hourly, and with a spatial resolution equal to that given by the carrier's antennas, four spatio-temporal points are enough to uniquely identify 95% of the individuals."