It's interesting to see who is on board, and who isn't. (on further review, some companies only provide an AAAA record for their "www" domain. Both www.bing.com and www.microsoft.com resolve, but do not respond to ping.)

  #ping6 google.com
  PING google.com(lga15s29-in-x01.1e100.net) 56 data bytes
  64 bytes from lga15s29-in-x01.1e100.net: icmp_seq=1 ttl=55 time=1.81 ms

  #ping6 facebook.com
  PING facebook.com(www6-10-08-prn1.facebook.com) 56 data bytes
  64 bytes from www6-10-08-prn1.facebook.com: icmp_seq=1 ttl=54 time=69.9 ms

  #ping6 linode.com
  PING linode.com(2600:3c00::12) 56 data bytes
  64 bytes from 2600:3c00::12: icmp_seq=1 ttl=51 time=47.8 ms

  #ping6 softlayer.com
  PING softlayer.com(2607:f0d0:1000:11:1::4) 56 data bytes
  64 bytes from 2607:f0d0:1000:11:1::4: icmp_seq=1 ttl=43 time=46.6 ms

  #ping6 xkcd.com
  PING xkcd.com(2001:4830:120:1::2) 56 data bytes
  64 bytes from 2001:4830:120:1::2: icmp_seq=1 ttl=52 time=20.7 ms

  #ping6 www.yahoo.com
  PING www.yahoo.com(r1.ycpi.vip.nyc.yahoo.net) 56 data bytes
  64 bytes from r1.ycpi.vip.nyc.yahoo.net: icmp_seq=1 ttl=59 time=1.55 ms

  #ping6 www.rackspace.com
  PING www.rackspace.com(2001:4802:821:c011::f5:147) 56 data bytes
  64 bytes from 2001:4802:821:c011::f5:147: icmp_seq=1 ttl=42 time=9.20 ms
  
  #ping6 www.amazon.com
  unknown host

  #ping6 engineyard.com
  unknown host

  #ping6 heroku.com
  unknown host

  #ping6 news.ycombinator.com
  unknown host

  #ping6 time.com
  unknown host

I can reach both www.bing.com and www.microsoft.com, but the latter seems to be blocked by a firewall rule:

  ~$ ping6 www.bing.com
  PING www.bing.com(2001:5002:200:2::c30a:b58) 56 data bytes
  64 bytes from 2001:5002:200:2::c30a:b58: icmp_seq=1 ttl=58 time=321 ms
  64 bytes from 2001:5002:200:2::c30a:b58: icmp_seq=2 ttl=58 time=234 ms

  ~$ ping6 www.microsoft.com
  PING www.microsoft.com(2a01:111:2004:3::13) 56 data bytes
  From 2a01:111:0:2::52 icmp_seq=1 Destination unreachable: Administratively prohibited
  From 2a01:111:0:2::52 icmp_seq=4 Destination unreachable: Administratively prohibited

I'm running a openvpn tunnel to a vps on a datacenter that provides me ipv6, so all my machines have a ipv6 address thanks to radvd.

postgresql.org seems to have the AAAA record.

My linode seems to have an ipv6 address, but I can't ping6 any of the hosts you mention above from it. Haven't investigated beyond that.

My home router doesn't seem to support ipv6. I think my ISP (comcast) does though.

Are Google ranking websites with ipv6 higher in search rankings?

It makes sense that they should, so ipv6 users can be sure they will get the best ipv6 experience of the links presented.

Almost no one has ipv6 only, as the vast majority of the web doesnt work yet, so it is not an issue yet.

Facebook got this:

  facebook.com [2a03:2880:10:1f02:face:b00c:0:25]

So, who else have got fancy IPv6 addresses?

Sprint.net has 2600:: (that's their entire address)

Last year Cisco had :c:15c0:d06:f00d in their IP but I think they've changed it.