It's a stark new warning for nearly all iPhone users, as Facebook has suddenly been caught "secretly" collecting sensitive data without anyone realizing it. To make matters worse, there is no way to stop this particularly aggressive tracking except by deleting the application.
Facebook still uses metadata from your photos and IP addresses to get location data, even if you update the "never" setting to track your location. Facebook acknowledged the gains but declined to explain why it was wrong for users to disable location tracking.
Now suddenly security researchers are warning that Facebook will go a step further by using the accelerometer to track a steady stream of movement on your iPhone, which could easily be used to monitor your activity or behavior when in particular places, or when interacting with apps and services. Worryingly, the data can even match you with people close to you, whether you know them or not.
As with photo location data, the most serious problem here is the absolute lack of transparency. You do not receive a warning that this data is being traced, and there are no Settings to enable or disable tracing; In fact, there doesn't seem to be any way to turn this feature off and stop Facebook(literally) from continuing.

Researchers Talal Haj Bakry and Tommy Musk warned: "Facebook is reading accelerometer data all the time. If you don't allow Facebook to access your location, the app can still deduce your exact location only by grouping users who match you to the vibration patterns recorded by the phone's accelerometer."
Researchers say the problem affects Facebook, Instagram, and WhatsApp, although it is possible to disable the feature when using WhatsApp, and the platform assures me that no data is left on users' devices."On Facebook and Instagram," Mesker told me, "it wasn't clear why the app was reading the accelerometer, and I couldn't find a way to disable it."That means you need to delete the app and access Facebook via a browser instead.
Given that Facebook dominates social media installations on the iPhone, this will affect almost all of the billion-plus iPhone users worldwide. Facebook confirmed to me, "We use accelerometer data for features like vibration reporting and to ensure certain camera features like 360-degree photos or camera panning."
"Even though the accelerometer data seems innocuous, it's jaw-dropping that an app can compose these measurements," Mesker said.”The app can learn the user's heart rate, movements, and even precise location. To make matters worse, all iOS apps can read sensor measurements without permission. In other words, users don't know when they're using the app if the app is measuring their heart rate."

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