"Facebook will also now know you visited a store based on a new feature that matches GPS, beacons, WiFi, radio signals, and cell towers with brick-and-mortar coordinates." [1] Basically harvesting data from your phone's sensors, where their app is installed.
And: "A new Facebook API, called the Offline Conversions API, works with a number of in-store sales systems from companies like Square and IBM to match their customer data with Facebook’s advertising data." [2]
(More science-fictiony: they could broadcast a local audio signal that fingerprints a particular location. Hmm, startup idea.)