Every generation that logs on thinks they invented the Internet. With Facebook, Mark Zuckerberg may actually have done it. His social network turns five today, and in that time, it has actually changed the world.

Not necessarily for the better, and certainly not in ways Zuckerberg originally expected or intended. The party line on Facebook is that the site is merely a reflection of the real world, not a substitute for it.

Facebook was born on the campus of Harvard University, and restricted to Zuckerberg's fellow students there. Every user knew at least that much about anyone they encountered. Even as it spread to other Ivy League schools, then high schools and offices, and then the entire Internet, Facebook maintained that feeling of a close-knit community, where no one's a stranger. Fakesters are possible, but not common. Spam, that plague of most communications media which reach critical mass, is pleasantly rare. Facebook has more than 150 million users. And yet one's News Feed, the running summary of friends' activity on the site and on other Facebook-connected websites, still reads like a small-town newspaper. All told, users spend more than 3 billion minutes a day on the site.

The one problem: Facebook lacks a financial engine that's growing quickly enough to match its user base. According to insiders' estimates, every 1 million users add $1 million in capital costs, as the company stacks up servers in datacenters to fulfill demand. Five years after their founding, the revenue streams of Yahoo, Amazon.com, Google, and eBay were firmly established. While Facebook is selling actual ads, its formats are ever-changing and its targeting inchoate. But that's the least of Zuckerberg's worries, really. Wherever an audience gathers, advertisers find a way of paying to be in front of it. What he should be more worried about it: Making sure Facebook remains firmly tied to the real world, rather than a cyberspace unto itself.

The result is addictive — sharing that turns to oversharing, conversations that turn in on themselves, and Ouroboros-like parade of online activity that ends up being more compelling than anything happening in the real world. For some vulnerable minds with not much to report back on from the real world, Facebook turns into a more entertaining pastime than actually going out and living.

Another danger: Just as 24-hour cable channels transformed the news cycle, Facebook's 24/7 reportage on your friends' moods could transmit groupthink faster than ever, and in so doing, endanger the world economy, by spreading panic more efficiently than ever before. Computers have previously accelerated the process of computation. How will things look when society itself is what they speed up?

As Facebook's algorithm rains down scientifically precise stimuli, we are the rats in Zuckerberg's maze. And we grow ever more eager for the pellets of computerized friendship he delivers. Who knows where this experiment will end?

(Photo of Zuckerberg in February 2004 by Lowell Chow/Harvard Crimson)