According to , who based his numbers on a complex calculation of U.S. census data and the 100 to 200 of the most popular surnames, worldwide users hit 7.3 million on Sunday. That's up from 1.7 million users on July 4, which would represent an increase of 350% in just six days, Allen wrote.

"I project that Google will easily pass 10 million users [Tuesday] and could reach 20 million users by this coming weekend if they keep the available," Allen wroteon Google+. "As one G+ user put it, it is easy to underestimate the power of exponential growth."

In an email to Computerworld, Google would not say if Allen's numbers are accurate or even in the ballpark. A spokeswoman simply said the company has not disclosed any numbers.

But Ezra Gottheil, an analyst with Technology Business Research, noted that it's a testament to how interested people are in Google+ that someone would go to such lengths to come up with a user estimate.

The people on Google+ are giving that estimate a lot of attention. As of 11 a.m. EDT on Tuesday, Allen's post had 109 comments, and it had been shared 1,000 times.