Some popular images seem to load for me, but longer tail stuff doesn't. Maybe they accidentally flushed their edge cache and then their origin couldn't handle the load.
Considering the type of content Facebook, WhatsApp, and Instagram host, I'd imagine the cache hit rates to be pretty low for media. They'd be effective for JavaScript/CSS though, which might also be served from the same system
I compared a few celebrity's instagrams with accounts of friends with many fewer followers. Celeb's images loaded but less and less as you scroll back through time, friends images all 503'd. Still seems to be the case.
Instagram also doesn't seem to be serving hot and cold content from different hostnames/IPs for me, so it couldn't be that either.
Also check out the official status update at https://developers.facebook.com/status/issues/34337305659485... and https://developers.facebook.com/support/bugs/610822019411772...