Facebook has seen immense growth ever since it opened up its gates to the general public in 2006. However, 2009 has been a remarkable year for Facebook by all counts. The social network doubled in size in US in the past year alone, according to stats released by comScore.

Facebook recorded 111.9 million US visitors in December 2009, compared with 54.5 million visitors in December 2008. This propelled the site to become the 4th most visited web property, up from number 11 in 2008. Facebook now accounts for 7% of all time spent online by US visitors.

These stats are impressive enough to make everyone at Facebook and their investors proud of the company. Having a deeper look reveals that Facebook grew substantially across all performance metrics tracked by comScore. Unique visitors, page views, and total time spent all increased by at least double. Average minutes per usage day grew by 6% and average usage days per visitors saw a gain of 37%. The only metric that saw a decline was the average minutes per visit, which went down by 11%. This tells us that Facebook users are now visiting the site more often and spending less time per visit – as compared to less frequent but longer visits last year.

comScore is not the only web analytics company to report such shiny stats about Facebook, as HitWise and Compete chimed in with their own figures recently. Facebook was the most visited site in the United States on Christmas Eve and Christmas Day”, according to Hitwise. This was the first time in Facebook’s history that it became the most visited site in US, as it one-upped on Google. Earlier this month, Compete released its monthly traffic for December. According to Compete, Facebook’s US visitors jumped to 132 million in December, 2009.

One possible reason for Facebook’s tremendous growth could be the so-called Zuckerberg’s Law – which states that “Next year, facebook users will share twice as much information as they share this year, and next year, they will be sharing twice as much as they did the year before.” This is because, once a network is in place with active and engaged user base, the dynamics social interactions incentivize participants to share information more regularly, which in turn solicits more engagements from friends and family, thereby creating a vicious circle of user interactions.

Mark Zuckerberg, Facebook’s founder, presented the Zuckerberg Law at the Web 2.0 Summit in November 2008. Looking in retrospect, Facebook’s growth has traced the trajectory of his prediction. If the law is to hold in 2010 as well, we would very well see Facebook blast past 220 million user mark by the end of this year. I wont be amused to see this happen, however the one thing that Facebook dearly needs to figure out is a revenue model.