[cromd]

Couldn't one say the same of any brick and mortar store? If a shoe store is going to open in my neighborhood, for example, how much data do they need for it to be optimally located? I guess they could seek out the online browsing history of all the friends of all the people living within 1 mile from the shoe store and analyze it.

I don't doubt that billboards are placed with some careful thought, but tracking is certainly not "required" - some aggregate stats about an area should work, especially as there isn't much feedback on who saw the billboard and bought a product etc.