Research has shown that smartphone sensors leave trackable fingerprints. Due to imperfections in the hardware manufacturing process, it is possible to distinguish different devices just from sensor data.
The researchers tested more than 100 devices over the course of nine months: 80 standalone accelerometer chips used in popular smartphones, 25 Android phones and two tablets.
The accelerometers in all permutations were selected from different manufacturers, to ensure that the fingerprints weren't simply defects resulting from a particular production line.
With 96-percent accuracy, the researchers could discriminate one sensor from another.
"We do not need to know any other information about the phone—no phone number or SIM card number," Dey said. "Just by looking at the data, we can tell you which device it's coming from. It's almost like another identifier."
Maybe this explains why Facebook would be interested in a fitness app.