The geolocation functionality authorized by a user on their mobile device effectively allows Facebook to know where we are at any time; but the company does not restrict itself to this way of doing, on the contrary.
The phone we use to take photos or videos also embeds metadata, such as the date, time, and geographic coordinates. In some cases, the longitude and latitude values are precise enough to expose the exact location, close to the meter.
Facebook also detects our location based on our Internet connection, for instance when we access the network from home, a coffee shop, or a particular store.
Geotagging not only allows Facebook to build detailed profiles on its users, retailers also use it to influence their behaviour by sending them personalized notifications and showing them context-aware ads (such as when we enter a store).
Our everyday moves in the physical world reveal valuable truth about our real identities, such as our trips to a medical clinic, the places we prefer shopping, or the people we meet (secretly) on Friday nights.