Google’s latest social networking venture, Google+, has proven wildly popular. One estimate by Ancestry.com founder Paul Allen pegs the service at 9.5 million users as of Tuesday morning. By the end of the day, he believes the service will reach 10 million users — and then some.

By cross-referencing surname popularity (courtesy of the U.S. Census Bureau) with 100-200 sample surnames of U.S. users, Allen calculates the total percentage of the U.S. population registered on Google+. Then, he examines the ratio of U.S. to non-U.S. users — one U.S. to 2.12 international users, as of July 4 — to generate a worldwide user estimate.

His calculations, if accurate, have revealed some staggering statistics. Google+ had roughly 1.7 million users on July 4. By July 10, that number had grown to 4.5 million. Following an exponential growth curve, about 2.2 million people have joined Google+ over the last 32-34 hours. At that rate, the service could reach 20 million this weekend, speculates Allen.