Comment by Crosseye_Jack

> But it can't be done en-masse, against every citizen.

I took that to be in reference to using "shoe leather" to conduct surveillance / investigations rather than today's ability to "simply" query a database for such information.

For example, back in the day (get off my lawn) if a crime happened in say a park in the middle of the night, then police would have to conduct door to door questioning to see who saw what and who in the area around the time, this required boots on the ground eating up man hours, something that doesn't scale up.

These days they can ask Google for a list of all the phones in the area at the time and will either have names/addresses tied to the Google accounts associated with that data or have enough data to then query the cell operators for that information.