Paul Allen of Ancestry.com is using a surname analysis to calculate the number of users on Google+. He believes Google+ will hit 10 million users tomorrow:
My model is simple. I start with US Census Bureau data about surname popularity in the U.S., and compare it to the number of Google+ users with each surname. I split the U.S. users from the non-U.S. users. By using a sample of 100-200 surnames, I am able to accurately estimate the total percentage of the U.S. population that has signed up for Google+. Then I use that number and a calculated ratio of U.S. to non-U.S. users to generate my worldwide estimates. My ratio is 1 US user for every 2.12 non-U.S. users.
Imagine how many will join once it’s not an invite-only beta.