Apparently Facebook over IPv6 is working: https://www.v6.facebook.com

    % curl -I https://www.v6.facebook.com
    HTTP/1.1 200 OK
    Strict-Transport-Security: max-age=7776000
    X-Frame-Options: DENY
    X-Content-Type-Options: nosniff
    P3P: CP="Facebook does not have a P3P policy. Learn why here: http://fb.me/p3p"
    Pragma: no-cache
    X-XSS-Protection: 0
    Cache-Control: private, no-cache, no-store, must-revalidate
    Expires: Sat, 01 Jan 2000 00:00:00 GMT
    Set-Cookie: reg_fb_gate=https%3A%2F%2Fwww.v6.facebook.com%2F; path=/; domain=.facebook.com
    Set-Cookie: reg_fb_ref=https%3A%2F%2Fwww.v6.facebook.com%2F; path=/; domain=.facebook.com
    Content-Type: text/html; charset=utf-8
    X-FB-Debug: [REDACTED]
    Date: Thu, 19 Jun 2014 08:03:22 GMT
    Connection: keep-alive
    Content-Length: 48441

So I'm guessing it's related to their IPv4 infrastructure. Say, a load balancer?