Facebook goes even further, using the accelerometer on your iPhone to track a constant stream of your movements, which can easily be used to monitor your activities or behaviors at times of day, in particular places, or when interacting with its apps and services. Alarmingly, this data can even match you with people near you—whether you know them or not.
Just like the photo location data, the most serious issue here is that there is absolutely no transparency. You are not warned that this data is being tracked, there is no setting to enable or disable the tracking; in fact, there doesn’t seem to be any way to turn off the feature and stop Facebook (literally) in its tracks.
Researchers Talal Haj Bakry and Tommy Mysk warn that “Facebook reads accelerometer data all the time. If you don’t allow Facebook access to your location, the app can still infer your exact location only by grouping you with users matching the same vibration pattern that your phone accelerometer records.”
Source: Michael Tsai