That's not correct. Facebook only has IPv6 servers, they removed IPv4 from all servers some time ago. Internally they only use IPv6 (both on servers and on desktops) because they ran out of RFC1918 addresses!
This is quite a good presentation about their network structure: http://www.internetsociety.org/deploy360/blog/2014/03/facebooks-extremely-impressive-internal-use-of-ipv6/
They translate IPv6-only to Dual Stack (IPv6+IPv4) at the network edge for legacy IPv4 users.
Biting the hand that feeds IT
