Founder Mark Zuckerberg used The Facebook Blog to announce that the social media site would this week be passing 400 million users - which is more than the combined populations of the US, UK, Ireland and Australia.
A year ago, in February 2009, Facebook announced it had passed the 175 million active users mark and in December last year that stat reached 350 million - still 50 million shy of today's numbers.
A range of new navigation tools have been unveiled to coincide with the anniversary, each designed to improve the user experience and some in direct response to consumer feedback.
The physical changes to Facebook, which is available in more than 70 languages, include a drop-down menu on each member's homepage listing their newest notifications, requests and messages.
When a member writes on another's wall or tags them in a photo, that person will see a red bubble appear in the left hand corner of the page, which becomes a drop-down menu.
In the last year a raft of developments have come out of Facebook, not least the implementation of Facebook Connect on more than 80,000 websites since its general availability in December 2008.
Zuckerberg ended his blog post by saying: "After the launch we're going to celebrate with a Hackathon - an event where all of us stay up all night coding and building out our new ideas for our next wave of products for you."
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