[treesknees]

Yes and no. Yes they could technically hardcode an anycasted IP address, however it'd be less reliable. Also you'd run into issues with TLS certificates. It'd be very inflexible and would probably result in more outages.

But even if they did hardcode an IP, the underlying infrastructure for Facebook was also down not just DNS resolution of facebook.com. So even if the FB app didn't need to resolve a hostname, it would still be broken.