The Wall Street Journal reports that Microsoft Corp.’s $240 million investment in Facebook Inc. — a three-year-old company with more promise than profit — “represents a huge bet that the online advertising boom will continue and the popular social networking site will be among the biggest beneficiaries.”

The thing that has analysts in a tizzy is the sheer valuation of the deal. That $240 million is only buying Microsoft a 1.6% stake in the company, meaning that the deal ends up valuing all of Facebook at a staggering $15 billion.

“The high valuation for Facebook is the latest sign of a renewed exuberance in Silicon Valley over Internet companies with lots of users — even if those users haven’t yet translated into much revenue — and is reminiscent of the Internet bubble that ended in 2000,” the article said. “Microsoft and Facebook say the valuation is justified and that Facebook is starting to find ways to monetize its rapidly growing user base.”

Microsoft clearly needs a hook to jump-start its sagging online advertising push, and move away from the increasingly dated operating system-and-Office juggernaut. The latter is not going away anytime soon, but Microsoft sees the writing on the wall, and it’s online — advertising, mobile, Web video, online media, or otherwise.

Microsoft Bets On Facebook Stake And Web Ad Boom [WSJ]