It’s only been two weeks since the launch of Google+ , but already there are “millions” of users, according to chairman Eric Schmidt. Beyond that ballpark figure, Google has not disclosed any user or usage numbers.

But Ancestry.com founder Paul Allen has been publishing his own estimates of the growth of Google+ (on Google+ itself). His latest estimate is that Google+ will surpass 10 million users today, only two weeks after its launch. That estimate is up from 4.5 million on July 9 and 1.7 million on July 4. I’ve charted the growth above, with Allen’s numbers.

If these numbers are anywhere near accurate, Google+ could very well become the fastest growing product in the history of the Web. Then again, Google is simply turning on the product for the hundreds of millions of users it already has once they get an invite and sign up.

How did Allen calculate his user number? Leaning on his knowledge of demographics, he used a clever technique comparing a sample of a few hundred surnames from Google+ and comparing them to how representative those names are in the general population (based on U.S. Census Bureau data). He made some assumptions about the proportion of U.S. to international users and came up with his estimate. It’s unlikely that his estimate is completely accurate, but he should at lest be capturing the growth trajectory.