"This is an important milestone for all of you who have helped spread Facebook around the world. Now a lot more people have the opportunity to stay connected with the people they care about," Facebook founder Mark Zuckerberg said in a blog.
The popularity of the social network has snowballed over the past year and a half. In January 2009 the site had 150 million registered users and in just over a year, in January it hit the 400th million user mark. It's taken just five months to gain another million users, which means 8 percent of the world's population use the social networking site.
It's thought around 26 million Brits use Facebook, that's roughly a third of the UK's population.
In a bid to celebrate reaching the milestone, Facebook has unveiled a new app that it hopes will encourage social networkers to share their experiences on the site with other Facebook members.
Facebook Stories allow users to post their own stories of the social networking site in up to 420 character. The stories are then sorted into different themes and locations, which can be viewed by other users.
"Our mission at Facebook is to help make the world more open and connected. I could have never imagined all of the ways people would use Facebook when we were getting started 6 years ago," said Zuckerberg.
Zuckerberg recently told The Telegraph he believes the site will reach the billion users with no problems.
"If we succeed [in innovating and remaining relevant] there is a good chance of bringing this [Facebook] to a billion people... it will be interesting to see how it plays out."