Unless the operators were in direct contact with Facebook, it doesn't sound like a good idea. It's certainly not the job of the ISP to reduce an outage for FB. They also weren't sure if the outage would only be 5 minutes or 5 hours. Instead, ISPs should scale up and handle DNS traffic for outages like this. In this case, FB isn't the only company to learn a lesson or two around failure modes and how to improve in the future.