Unique in the Crowd: The privacy bounds of human mobility:

We study fifteen months of human mobility data for one and a half million individuals and find that human mobility traces are highly unique.

Wow – they know who I am just from my location patterns. No use signing in and out of different accounts on the phone! I guess the same would also be true for browsing from desktops, with the many parameters they can get from desktops when browsing websites.