Theoretically (i don't work at facebook) there's a number of ways you could go about it.

- Geocode down to the lowest applicable level of IP address

- grab it from the photo metadata

- Piggyback off of similarities to other people obtaining similar photos or activity

- grab it from other database sources that do have your location

- correlate it to your interactions with other hardware/infrastructure with known locations

- transitively deduce it from your friends and acquaintances who share their location and tag themselves and reply to your stories/photos

Theoretically you could try to train some neural nets on images themselves. Geoguessr.com is a fun game website where you can try to do so as a human, and you can usually get pretty damn close. Images themselves leak a tonne of information on time/place: clothing styles, fonts and languages on signs, shadows, light colour, fauna/flora, asphault + stone types, arichtecture and design quirks. In many ways the neural net might even have some advantages over humans, because not many humans memorise the thousands of minutae: eastern USSR uses this particular kind of road barrier and reflector type and this kind of font on its road signs.

But aside from a research activity, i imagine facebook already has the data it needs most of the time...

I once posted a shot including a friends daughter. FB tagged her mother before I shared the post. We were all creeped out.

Geo IP is a reasonable last resort; it'll usually show at least what city you're in.

The images you upload may have location data on them.

If you're using a wireless hotspot then your location is correlated to anyone who also used it while sharing GPS data.

And so on.

"If you're using a wireless hotspot then your location is correlated to anyone who also used it while sharing GPS data."

That's a clever one I hadn't thought about. I guess for a service like Facebook/Instagram/Google it can work decently well given the scale/adoption.

That's in fact how your GPS works most of the time. They triangulate it based on a scan of SSIDs assuming access points are usually fixed. Both Apple and Google use this method.

fun fact - I changed my hometown to Timbuktu on facebook. yet when I click on Why Am i seeing This Ad, I am told it is because the advertiser wants to reach people who live in (not are in or have been in) my actual home town. huh.

Honest question, I don’t know the iOS APIs very well, can they (if you have given the app permission before) check geotags on your photos without you explicitly uploading that photo?

I believe if you give access to "Photos" on the device, they can. Otherwise by giving GPS/Location permission - no. They are two separate things.