After opening its doors to the public last Tuesday, Google+ may now have more than as 43 million users.

So says Ancestry.com co-founder and Google+ unofficial statistician Paul Allen, who's been keeping a running tally of how big the social network is growing.

Compiling his regular series of stats based on surnames used around the world, Allen said last Thursday that Google+ witnessed a 30 percent jump in users in the two days following its official opening to all users on September 20.

In just those two days, the growth rate skyrocketed to that only seen during the first week of the network's "field test" in late June, according to Allen, who's unrelated to the Microsoft co-founder of the same name.

Projecting the figures over the past few weeks, Allen's model came up with 28.7 million users on September 9. Last Thursday morning, that number had swelled to 37.8 million, with most of the growth coming from the prior two days. And accounting for a fudge factor and other elements, he came up with an estimate as of last Thursday of 43.4 million users.

Over the past couple of months, Allen has been busy projecting the ongoing growth of the social network. From 1.7 million users on July 4, about a week after Google+ kicked off, Allen's estimate rose to 4.5 million on July 9, 10 million on July 12, and 18 million on July 20.

Google itself has generally been mum about user counts as its social network has grown, though it did cite 10 million users on July 14, not too far off from Allen's projection at the time.

In response to the latest user estimates, a Google representative told CNET that the company isn't commenting on Allen's statistics and that it hasn't announced any new metrics since July 14.

Despite the ongoing growth in overall users for Google+, a report from Experian Hitwise earlier this month claimed that the number of weekly visits to the service has been dropping since mid-July.