Facebook has just turned six and it's proving quite an eventful birthday. CEO and founder Mark Zuckerberg has just posted a teary-eyed message on just how important the social network is and just how many lives it irrevocably changed, presumably for the better. But sprinkled in there was a piece of info much more important than all the touchy-feely stuff startup founders just love to talk about, though no one can match Twitter cofounder Biz Stone at that, not even Zuckerberg, it turns out that Facebook has now reached 400 million users.

Probably the most unsurprising announcement of this sort in quite a while, but the number is now official. Facebook hit 300 million users just five months ago and 350 million just a couple of months back. For those wondering just where the upper limit for user growth is, we have one answer, it's above 400 million.

And, by the looks of it, way above, as there's no sign that Facebook's growth is stopping any time soon. 500 million users is now just around the corner and after that it's anyone's guess. It's going to have to stop someday, there are only so many people online, above 1.7 billion by some numbers.

One thing is for sure, Facebook is now at a completely different level than any other social network, in fact it's bigger than all the big social networks, MySpace, hi5, Twitter, LinkedIn etc., put together. And if the current trend continues, it's going to become the biggest web site on the planet, overtaking Microsoft, Yahoo and even Google sites, in the not so distant future.

In the meantime, Facebook is celebrating its sixth birthday surprisingly like any other six-year-old out there, wearing its brand-new clothes, err design which just went live for a significant number of users, at least 80 million, and will be made available to anyone soon. The design also represents a change of course for the social network, one that began a few months back when it decided to do a 180 degree turn and encourage people to be more open with each other, or rather with the world wide web. But if this new course turns out to be even remotely as successful as the site has been so far, Facebook will become the biggest thing the Internet has ever seen.

"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 wrote in his heart-warming message.  "Whether in times of tragedy or joy, people want to share and help one another. This human need is what inspires us to continue to innovate and build things that allow people to connect easily and share their lives with one another," he added.