The frenzy over the launch of Google+ has slowed down in the past couple of days, compared to an estimated 2 million users joining every day in the past week, suggests an analysis by Paul Allen, co-founder of Ancestry.com.

Allen's statistics could have an error of 30 to 35 percent as his measurement is based on randomly selected surnames of Google+ users every day.

If Google+ would have continued with its initial rate of growth, it would have reached 20 million users by Sunday night, which Facebook had attained after three years of its launch.

Facebook was started in February 2004 and by December 2004, it had reached 1 million active users. It reached 20 million in April 2007 and soon shot up to 90 million active users in 2008, according to Comscore data. With its expansion in terms of translation into various languages and penetration into multiple developed and developing markets, Facebook has seen huge success in 2009-2010.

Facebook has enjoyed its crests of popularity in the past two years, leaving its contenders way behind. As of July 2011, more than 6 million users in the U.S. have left Facebook in a month's time, though the social networking site still has a user base of more than 750 million.

Despite the slight decline in the last two days, Google+ has reached a total user base of 18 million.

However, Google has not started marketing the service on other properties yet and is still focusing on the feedback of the users to quickly improve the service.