I think we're tracking the wrong number. We should not be tracking adoption, we should be tracking it's integral value. Not the current position of IPv6 adoption but abasement of IPv6 adoption.
(As a side note, Google and APNIC disagree significantly on China's deployment percentage -- 6.5% vs 52% -- so there's actually something like another half a billion v6 users not represented in Google's stats which would push the world percentage from ~48% to ~56%, and that "x years ago" stat to 10 years ago.)