Google+, Internet search giant Google's social networking service that aims to challenge Facebook, will have 400 million users by the end of 2012, Ancestry. com's founder Paul B. Allen estimated.

According to Allen, who describes himself Google+'s informal statistician, Google+ is gaining around 625,000 new users per day and thus will be able to finish 2012 with 400 million users.

Announcing his estimates, Allen wrote in a blog post, "There is no question that the number of new users signing up for Google+ each day has accelerated markedly in the past several weeks."

In October this year, Google had announced that its social networking platform had 40 million users around the globe. On the other hand, Facebook, which has plans to float an initial public offering (IPO) next year to lift up to $10 billion, has more than 800 million users.

Around a week ago, ComScore released figures showing that Google+ had grown to 67 million monthly unique visitors during November. But ComScore's figure represented only the number visitors.

Google's social networking service may be benefiting from the popularity of its Android mobile OS that makes it easy to sign up. New users are also inviting their family members and friends to join the platform, providing a further boost to its growth.