It is actually quite easy to do. ISPs and companies that advertise their own IP space to the Internet routers rather than rely on ISPs would have the ability to do it. Large companies advertise their own Internet IP space to the Internet, like mine. The Internet routers are actually quite trusting when it comes to believing routes that are learned from other Internet routers. It basically works like this, a router will announce (using the Border Gateway Protocol, BGP, on the Internet) if you want to get to X.X.X.X/X address space, send your traffic to me. The Internet routers, using BGP to communicate with each other, will figure out the best way to get traffic from Point A to Point B. Sometimes Internet routers have policies which limit which routes they learn from others, kind of like guardrails from being too trusting. Now the issue is the length of time Facebook and its entities have been offline. This is fairly unheard of. My best guess is Facebook took itself offline; stopped advertising its own address space.