Facebook has marked its sixth birthday by revealing it has more than doubled its number of users in the last year to 400 million.

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 figure hit 350 million – so 50 million more have joined its ranks in just two months.

The anniversary has been marked by the launch of a series of new navigation tools for the site, “including a couple that people have requested a lot”, according to Zuckerburg.

The physical changes 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 it was made generally available 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.

“We look forward to continuing to serve you for many more years.”