Interestingly, my comment's getting down-voted without refutation. C'mon, let's talk :)
Anyway, no, I don't know if FB can do that at scale.
What I do know is that sites, especially complex ones like FB, like to track user interaction to evaluate UX (hover targets, click targets, time it takes to find call-out etc).
If I were a data science type at FB, and knew FB was collecting that stuff, I think I'd like to find out what other questions I could answer with it.
Or more banal - did you use your laptop as a wifi endpoint and connect your phone & WhatsApp to it?
