Interestingly, Pokemon GO only prompts players to scan a subset of the Points of Interest on the game map. Players can manually choose to scan any POI, but with no incentive for those scans I'm sure it almost never happens.
> Today we have 10 million scanned locations around the world, and over 1 million of those are activated and available for use with our VPS service.
This 1 in 10 figure is about accurate, both from experience as a player and from perusing the mentioned Visual Positioning System service. Most POI never get enough scan data to 'activate'. The data from POI that are able to activate can be accessed with a free account on Niantic Lightship [1], and has been available for a while.
I'll be curious to see how Niantic plans to fill in the gaps, and gather scan data for the 9 out of 10 POI that aren't designated for scan rewards.