said by file:Facebook launched a bit early and this was only within the last two weeks to allow third party developers to work with said v6 support and v6 enable their apps.
Two major ones have embraced IPv6 - Facebook and Youtube. I access both over IPv6 these days without realizing it.
YouTube is using a whitelist so who has access to YouTube even if they have v6 access now is quite limited. So at the moment you have to be using a resolver which has been whitelisted by Google and if that is the case you would have access to almost all of Google's web properties over v6 and not just YouTube (.e.g. GMail, Google search, Blogger, etc.)