It was announced this morning that Microsoft has agreed to pay US$240 million to purchase a 1.6 percent stake in Facebook, the growing social networking website. The purchase values the three-year-old company at a mammoth US$15 billion.

In return for its investment, Microsoft will be responsible for selling advertisements which appear on the Facebook site globally and splitting revenue (Microsoft had previously agreed to sell Facebook’s US-advertisements until 2011).

The deal is limited to only advertising and the small equity stake, with CNet noting “there won’t be any integration of Facebook into Microsoft services, nor will Microsoft’s non-advertising properties–like Windows Live Messenger, for example–be worked into Facebook.”

The biggest winner appears to be Facebook’s wunderkind finder, Mark Zuckerberg, whose 20 percent holding in the unlisted company is now valued at US$3 billion. Zuckerberg recently turned 23 and has a business card which reads, “I’m the CEO, b-tch”.

Many question whether Microsoft has overpaid for its stake, given Facebook has revenue of only US$150 million. However, Microsoft isn’t that stupid. And Facebook isn’t pets.com. Microsoft’s investment was made probably more due to fear than anything else. Fear of missing out on the latest craze. Fear of being beaten by those Google guys, again.

Facebook has been mooted as becoming an on-line operating system. As the New York Times noted:

The high valuation [paid by Microsoft] represents a belief that Facebook is creating an important new operating system — one that exists on the Web instead of on personal computers. In May, it opened its platform, inviting other companies and third party developers to create tools for the site and share in the advertising revenues.

The move unleashed a flurry of activity around the social network. More than 4,000 applications, like games and music-sharing tools, have since been created for the site, which in turn has accelerated Facebook’s membership growth. The company says it now has more than 42 million members and will exceed 60 million members by the end of the year.

If Facebook fails to live up to the hype, it probably won’t really matter for Microsoft anyway. Microsoft’s investment of US$150 million will barely rate on its balance sheet. Despite reducing its cash pile in recent years, the Redmond, Washington-based company still has cash and short term investments of approximately US$29 billion (down from around US$60 billion in 2004).

Microsoft shares were up 1.13 percent after the announcement to close at $31.25 per share (valuing the company at US$293 billion) on a relatively low forward PE of 16 times 2008 earnings. The company remains well short of the peaks reached in the dot.com boom (Microsoft shares his $58.9 in 1999).

The 23-year-old Zuckerberg will leap on the Forbes 400 list of Richest Americans, in between 61-year-old Donald Trump and 79-year-old T Boone Pickens.

Analyst Greg Sterling told Forbes today that:

 … you can see the Facebook scenario in one of two ways. Either they’re very shrewd and clever or incredibly greedy… but either way their strategy seems to have been validated.