That's interesting, but the site isn't being served over IPv6. The IPv6 page is just a redirect to the IPv4 page, so you still can't use it if you're IPv6-only. (And HTTPS connections to https://[2a03:2880:10:1f02:face:b00c::25] fail.)
The site is being served over ipv6, they are just redirecting you because your Host header doesn't match what they want.
This works just fine:
curl -g -H "Host:www.facebook.com" -H "User-Agent: Mozilla/5.0 (Macintosh; Intel Mac OS X 10_7_4) AppleWebKit/536.5 (KHTML, like Gecko) Chrome/19.0.1084.46 Safari/536.5" "http://[2a03:2880:10:1f02:face:b00c::25]
Ah, so they are getting ready for World IPv6 Launch, but not entirely there yet. I didn't flip to my IPv6-only network to check it. Thanks for noting this!
I just noticed yesterday that nmap does the same with their website, that's why I thought to check. http://[2600:3c01::f03c:91ff:fe96:967c]/ is web.insecure.org but it just redirects to the IPv4 version.
