Ten million users is a mere drop in the bucket compared with social networking giant , which confirmed last week that it has surpassed 750 million users.

However, if Allen's figures are close to accurate, it means that Google+ is seeing some very strong growth. And that bodes well for a social network that has a decidedly uphill battle in taking on Facebook.

"I don't think it's impossible for Google+ to have hit those numbers," said Gottheil. "Given its high profile and the now open invitations, it could well be true."

Bill Gross, the founder and CEO of IdeaLab, a business incubator based in Pasadena, Calif., also got attention on Google+ when he predicted this week that the social network will go from 0 to 100 million users faster than any other service in history.

"People are engaging with it like crazy," said Gross in a wall post. "Now it's not completely fair, since when Facebook started, and when Twitter started, etc. those were tiny companies, and Google is huge. However, the product is extremely well executed, and a lot of people are smitten."