[wmf]

The risk is that ~0.001% of computers won't be able to access Facebook or Google for one day because they think they have working IPv6 but they actually don't.

[zaphoyd]

The risk is much smaller today than it was when the ipv6 whitelists for google, etc were originally set up. All patched modern operating systems and browsers have corrected the behavior that originally caused this (preferring automatic v6 tunnels over native v4) so that the only brokenness remaining in theory is outdated software and improperly configured manual tunnels.

See http://fud.no/ipv6/ for more information on measurements of this issue as browsers and oses fixed this problem.

[torme]

Since I can't access the v6 version of facebook, does that mean that I'm in the 0.001%?

[wmf]

Not necessarily; if your computer doesn't have IPv6 and knows that it doesn't have IPv6 then you're going to be fine.