So it actually turns out that on Android if you opt out of ads personalisation, the app still sends the advertising ID, but also sends in the JSON "advertising_

So it actually turns out that on Android if you opt out of ads personalisation, the app still sends the advertising ID, but also sends in the JSON "advertising_tracking_enabled: false", which is not so reassuring. See Privacy International's talk here: https://media.ccc.de/v/35c3-9941-how_facebook_tracks_you_on_android#l=eng&t=1271 https://media.ccc.de/v/35c3-9941-how_facebook_tracks_you_on_...