They’re tracking people using their phone’s advertising ID. Here’s an EFF post about resetting it.

On newer versions of iOS apps have to ask for permission to access your device’s advertising ID; Facebook was very unhappy about that. Turning off Settings -> Privacy & Security -> Tracking -> Allow Apps to Request to Track will (should?) keep apps from getting your advertising ID. I’m not sure if Android has anything like that, but Google is an advertising company so my guess is No.