facebook started migrating to ipv6-only datacenters in 2014 or so. i think all of them are converted at this point. they only support legacy ip at their network edge, & use siit (iirc) to facilitate access.
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.
I could see doing IPv6+IPv4 in a corporation and terminate everything on load balancers, allowing anything behind the LB to be any combination of IPv4/IPv6. But IPv6 only? I don't see any big companies doing that in my lifetime.
My ISP obtained some IPv6 space but have not deployed it yet as they are still trying to decide how to subnet and bill for it so I currently have some static IPv4 addresses. I am on a tiny ISP, so tiny all their C-Levels phone numbers and email addresses are still in whois. Most of the US ISP's doing IPv6 also do IPv4 even if sometimes it is CG-Nat.