[LinuxBender]

The legacy IP at their edge is what I meant by terminating IPv4+IPv6 at the load balancers.

The tricky part is that almost all datacenters today need to talk out to other datacenters. Not all of them use IPv6 which means they will still need some way to speak IPv4 until all their 3rd party data processors are also doing IPv6 and for Facebook I happen to know that is a lot of 3rd parties. If they are truly IPv6-only in the datacenter then they would have to forward-proxy all outbound connections through something with an IPv4 address on the edge as well and that can not go away until all their partners and vendors are also purely IPv6.

[Dagger2]

You can do that with NAT64, which you can run as a service in your datacenter rather than needing to deal with v4 throughout it. (In fact it doesn't have to be run in your datacenter -- it could be outsourced to somebody else, which will probably make sense eventually as v4 use dwindles.)