> Is it even possible to do this on an iOS device that's on 4G

No.

> Is it even possible to do this on an iOS device that's on [...] someone else's wifi

Yes, since you can do it on your device, and do not have to do it on the router. Drawback is you have to do it for each Wifi network anew.

> Do iOS devices have the same "leak"

Yes, there is nothing that prevents apps from phoning home (or phoning every one of a dozen data collection "partners")

You either need to control the mobile side of things and never connect to unrestricted Wi-Fi or use Apple Configurator to create a profile for an always-on VPN to a place you control where you can apply the restrictions.

I am unable to verify this right now for the obvious reason, but facebook operates on ASN gAS32934[0]

So you can ask https://www.radb.net/ for the IP addresses that are associated with this AS and, after a quick manual sanity check, insert it into the firewall of your choice. For example:

  whois -h whois.radb.net '!gAS32934'|  tr ' ' "\n" | sed 's/^/saddr /'| sed 's/$/ DROP;/'

[0] https://www.facebook.com/peering/