Never having explicitly queried fb.com before I never noticed how they (face:b00c) got clever with their IPv6 address:
2a03:2880:f1ff:83:face:b00c:0:25de
2a03:2880:f1ff:83:face:b00c:0:25de
You know (or could find out) the hashing algorithm. I’m sure you could come up with a model estimating “pollution” based on hash rate, kWh per hash, “greenness” of the energy source, and estimated hashes until a prefix collision.
Excited to see what you come up with.