Google Inc. is adding 625,000 users a day to the Google+ social-networking service, which may total 400 million members by the end of next year, according to an independent analysis of its growth.

The site's popularity has accelerated in recent weeks, with almost a quarter of its total user base joining in December alone, said Paul B. Allen, founder of Inc., who tracks the numbers as Google+'s "unofficial statistician."

Google, the world's largest Internet-search company, aims to challenge the social-networking supremacy of Facebook Inc., a site with more than 800 million members. Google+, which lets users organize their friends in circles, was introduced earlier this year as a test project, then opened up to the general public in September.

Google+ may be benefiting from the popularity of Google's Android mobile operating system, which makes it easy to sign up.

As the service gains traction, more people will invite family and friends to join, further accelerating its growth, Allen said in a Google+ posting. He works at, a company he helped start in 2006 with other cofounders.

Katie Watson, a spokeswoman for Mountain View, Calif.- based Google, declined to discuss the current user numbers. The company last gave an update during its October earnings conference call, when Google+ had more than 40 million users.

Facebook, meanwhile, is said to be preparing an initial public offering, helping generate funds for its own expansion.

The company is considering raising about $10 billion in the IPO, which would value Facebook at more than $100 billion, a person with knowledge of the matter said last month.