Facebook may have grabbed most of the headlines last week — but at the same time, behind the scenes, young and scrappy rival social network Google+ was grabbing a record number of new users.

According to a post by Paul Allen, Ancestry.com co-founder and unofficial Google+ statistician, Google+ is up to an estimated 43.4 million users — and roughly a third of them have been added in the few days since Google+ opened itself to the public.

This isn't Allen's first foray into Google+ numbers. His system for counting users first gained credibility in early July, when he estimated the service had 10 million users — and Google CEO Larry Page said the same thing in a call with investors two days later. He has since pegged Google+ user numbers at 18 million, then 28.7 million in early September. But the site has not seen this kind of growth since its beta launch.

"The growth rate has skyrocketed to rates we only saw during the first week of its field test," writes Allen. "Back when it had a small number of users to begin with."

Indeed, Allen took his snapshot of the service last Thursday — so those 30% extra users arrived in the two days since the service officially opened its doors. We await with bated breath updated figures for the first week of Google+'s public existence, not to mention any official word on numbers (especially since Allen admits he had to guess at some of the numbers). We're guessing Facebook is going to be watching closely too.

[via CNET]