Facebook has yet another reason to celebrate about this week apart from celebrating it’s 6th anniversay.

According to Facebook founder Mark Zuckerberg, the No. 1 networking-site is hitting 400 million membership this week.

“Today we’re celebrating our sixth birthday, and this week there will be 400 million people on Facebook. Just one year ago we served less than half as many people, and thanks to you we’ve made great progress over the least year towards making the world more open and connected” Zuckerberg said in a blog post.

“Facebook began six years ago today as a product that my roommates and I built to help people around us connect easily, share information and understand one another better. We hoped Facebook would improve people’s lives in important ways. So it’s rewarding to see that as Facebook has grown, people around the world are using the service to share information about events big and small and to stay connected to everyone they care about.” Zuckerberg said in the blog.

The staggering usage growth of Facebook continues unabated as the company achieves yet another milestone – 100 million U.S. unique visitor in November. Accoridng to comScore, this is the first time the social networking site has exceeded that monthly mark.

Just recently, Aol celebrated its re-emergence as an independently-traded company. It was the fourth largest Website in the U.S. until last month. Comscore data for November, 2009 shows that Facebook went past Aol with 102.9 million unique visitors in the U.S., against Aol’s 99.7 million. The other leaders are Microsoft, Yahoo, and Google with 130, 160 and 168 million U.S. visitors respectively.

On a global basis, Facebook already passed Aol back in Febuary 2009 to become the fouth largest site worldwide. With the current pace of Facebook, it might not take long to catch up Microsoft, Yahoo, or Google.

How long do you guys think Facebook will take to become the largest Website in U.S.?

[Source: Washington Post]