I don't think Facebook's apps work when there's no access to DNS. At least it didn't seem like it when I was working to keep that capability for WhatsApp as it moved into FB datacenters.

I don't think very many other applications will work without DNS either, although I never did much competitive testing.

Facebook could (relatively) easily modify their system to do its own IP handling; they probably have enough money to rebuild the entire stack.

Sure, they could, but at least while I was there, there was no interest in doing it, and amazement than anyone else would want to (and push back on declaring at least a handful of IPs as stably allocated enough to be included in app downloads).

Telegram does AFAIK.