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It's like stalking with time travel.
If everyone Stravas their commute then using flyby you can get the details of anyone you saw on your commute by replaying it in flyby mode and looking at their Strava id. This is because Strava overlaps all the commutes of everyone you were near and since lots of Strava users don't set up privacy zones you can then see where they live and where they work. What bikes they own and which ones they ride most often.
What is flyby?