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Hmmm "One current employee asked that a WIRED reporter turn off his phone so the company would have a harder time tracking whether it had been near the phones of anyone from Facebook."

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This is well known.

How is this done? Does the Facebook mobile app record all location data? Or does it record all nearby bluetooth devices?

I don't know how the details of this is done. However, you should assume that every piece of that that the Facebook app has access to, it will relay back to HQ. It will be stored and correlated with other datapoints that they have of you. They will know who you encounter as you go about your day.

I'm super surprised that they haven't made a dating app out of this.

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