Just how many people may be hanging out on Google+?

Paul Allen, founder of Ancestry.com, said there are more than 4.5 million people using Google+, leading him to believe that this social site is Growing Like Crazy.

On his Google+ profile, Allen wrote that he will soon publish his latest research on how many Google+ users there are so far.

Allen's model is based on U.S. Census Bureau data on how many people there are in the United States with each surname.

For example, Allen's data showed that there were 12 people with the last name Tat on Monday, and by Saturday there were 51 people. There were five people with the last name Tomczyk on Monday and 20 on Saturday.

I've described before the shortfalls of this model for calculating non-U.S. users, Allen wrote. But I haven't seen a better methodology from anyone else yet about estimating Google+ growth, so I'll continue to work on this one.

Allen said he's updated about a quarter of his surname counts, but preliminary calculations are suggesting that since Monday, Google+ users more than doubled, growing from 1.7 million to 4.7 million.

Again, this is preliminary but by Monday morning I should have 4x as much data to improve the estimate, Allen wrote.

Google+ hasn't released any information on the number of people in its Google+ trial.

However, its competitor, Facebook, currently has 750 million users, as confirmed by Mark Zuckerberg last week.