As Facebook turned five years old last week, it marked the passage with a cute retrospective of its various profile page designs and a subdued message from founder and CEO Mark Zuckerberg, suggesting that members give virtual gifts to their friends.

Facebook is a 21st century success story and is the giant in the room when it comes to social networking. It out-cooled MySpace, out-usefulled LinkedIn, and outlasted Friendster. In making a recent investment, Microsoft valued the company at $15 billion. D00d.

Facebook has one tiny problem: it has not yet developed a working revenue model. And its window of opportunity to do so will not be open indefinitely.

In a recent article by Agence France-Presse:

Facebook, unlike other Web giants such as Amazon, eBay, Google and Yahoo!, is yet to prove how it is going to translate traffic into cash. . . . “There’s no significant visible source of revenue other than investors,” said Silicon Valley analyst Rob Enderle of the Enderle Group. . . . “There’s a lot of potential there but they’re still kind of living in this dotcom mindset where a business plan doesn’t make a difference,” he said. “And as we saw with the dotcoms, that has a very unfortunate end to it.”

I love Facebook and I sure hope it figures out how to make real money. But no institution lasts forever. The days of GM, IBM, GE are numbered — as are the days of current behemoths Google, Microsoft, and others.

What will replace them? There is no way of knowing. Maybe Yahoo! will reinvigorate itself and kick Google’s butt. Maybe Canonical Software will convince a tipping point’s worth of people they don’t need to pay for an operating system.

One thing’s for sure, and that is that whatever our predictions are now — they are as likely to be wrong as right.

For now, I am just left wondering: what’s next?