I think we may be talking about two different sets of numbers. It sounds like you're quoting GOOG's penetration into US (or OECD) total population. (I'm guessing from the percentages.)
I'm saying that the aggregate number of people using the Internet has grown much, much faster. I don't have exact numbers handy, but it's certainly possible that the total Internet population has grown more than 100% over this period.
You also raise an interesting point about frequency of usage, which would also contribute to valuations.
In 2012 it doesn't require huge leaps to see how a company Pinterest rapidly could get to revenue numbers that support this valuation. At YHOO's current revenue-multiple-margin mix, this would imply revenues of around $400m-$500m. I don't know what their revenues are now, but given their growth and reach I'd be surprised if investors didn't pay a higher multiple for Pinterest than for YHOO.