[samoa42]

suggests that ipv6 is here to replace the rfc1918 space but "the internet" largely remains ipv4.

the high v6 penetration in mobile networks likewise suggests that smartphones are generally not a part of the network as much as dedicated leaf nodes that require no inbound connections.

[p1mrx]

There's a difference between "IPv4 is only present at the edge" and "only IPv4 is present at the edge".

I think Facebook is doing IPv4+IPv6 at the edge, and IPv6-only internally.

[ArchOversight]

You are correct. Facebook has done multiple presentations where they showed how they have a IPv4 load balancers/translation layer at the edge that converts everything to IPv6... but native IPv6 is well, just that, native.

There's no translation layer needed.