Today I've received no fewer than six invitations to join Facebook. What's Facebook? Where've you been Grandad? Facebook is the social-networking behemoth that's gobbling up the world. And, it seems, Facebook has finally entered the sweaty circle of trust that I like to call: 'Matt's friends'. I'm clearly not the only one, either.

Facebook, the university spare-time creation of archetypal sweatpant-wearing US college dropout Mark Zuckerberg, consists of 47,000 college, school, employee and regional networks. Some of the biggest firms in the US have Facebook networks. And if you think that's impressive, try this: Facebook handles some 600 million searches and more than 30 billion page views each month. (That's even more than PC Advisor.)

In fact, Zuckerberg's Facebook is the sixth most-trafficked site in the US. It may even be the biggest photo-sharing site on the web (according to comScore, more than 1 billion photos are stored on the site, and a mighty 6 million are uploaded each day).

And this is one phenomenon that is so not over - Facebook claims its user base is growing 3 percent week on week. Mark my words, we're all about to live in a Facebook world.

So what's so special about Facebook? On the, ahem, face of it, it's little more than a less garish MySpace with no ads. Which may be significant.

Facebook's demographic makes for interesting reading - 100,000 and more of its users are of pensionable age in the UK (by which I mean 64 and over. If you are reading this in the future, 'retirement' used to be what you did when you finished working before you popped them.)

The biggest cohort of Facebook users (around 3 million) are aged between 25 and 34 years. That is, coincidentally, my agegroup and that of many of my friends. It's also, I'd wager, an older age group MySpace.

But the relative lack of ads may be more significant still. Zuckerberg has, by common consent (no hard facts here) turned down offers for Facebook of $750m from Viacom and $1bn from Yahoo. But despite huge revenue predictions from all sides, no-one really knows if Facebook is making much in the way of hard cash.

So for all that Zuckerberg (who lives in a rented flat and wears sandals) sounds like an interesting chap, he may be destined to be known only as the man who turned down a fortune. Remember Friendster, anyone? You won't.