its not really all that "custom".... facebook have a shead load of IPv6 space, and based on what hurricane electric say (
https://bgp.he.net/AS32934#_prefixes6) most of it is registered in Ireland... their main prefix is a /32, so the first 2 groups of digits are given to them by the RIR (RIPE probably, since its Europe) and the rest is up to them... they can have anything in there in hex... I have seen traceroutes with dead:beef, deca:feba:d, etc... its up to the net admin to make it out how they want... what would be custom is if they had that at the start of their block...