Microsoft’s $240 million investment in Facebook could easily be interpreted as a sign that we are truly in the midst of Internet Bubble 2.0. After all, Microsoft’s acquisition of 1.6 percent of the company sets Facebook’s valuation at $15 billion, or about 100 times the company’s estimated ad revenues of $150 million this year.

In scrambling to rationalize the sky-high price tags being assigned to social networks such as Facebook, some experts argue that these sites--and even big search sites like Google--are the operating systems of the future. Because these companies have opened their inner workings enough to allow third parties to write add-ons, known as widgets, analysts draw a parallel between Windows or Linux and the applications that are created to work on the Web-based systems.

"Facebook really represents the new computing platform for this new age of computing, and I think any social application that is written in the future is going to have to take into account the Facebook model," Forrester Research analyst Charlene Li said.

But if Facebook is the new operating system, Microsoft’s investment seems more focused on the ads it can place before Facebook’s 50 million users than in the idea that the site is a threat to the Windows monopoly. By all indications, Steve Ballmer bit the bullet on the valuation because he couldn’t afford to miss winning a toehold in the young, fast-growing Facebook audience that is the Internet’s future.

But what about this operating system stuff? Do widgets make an OS? The idea is intriguing. In the short time since Facebook opened its robe, thousands of add-ons have been created to work with the site, from games to mobile applications that let you update your status (an important aspect of Facebook is your ability to let your friends know where you are and how you feel at any moment).

Now a large number of Facebook add-ons can be downright annoying–such as those that let you send “drinks” to your friends. No doubt, we’re headed toward a category we will soon call "widget spam." But some are actually useful. I like the Friends Wheel, which lets you see a visual map of the connections among your acquaintances.

But the real value of these widgets to Facebook is keeping users on the site. Advertisers value “face time,” and the longer users stay in Facebook, the more likely they are to see the ads that are flashed on the screen.

As operating systems go, Google may actually be closer to offering products that meet the classic definitions of applications. Google’s word processor, spreadsheet, and e-mail are a step toward challenging the Microsoft monopoly, but they don’t yet come close to offering the rich feature set of products that have been developed over nearly two decades. But here too, Google keeps chipping away: its iGoogle desktop lets each user choose what information and what widgets to place on the home page. You already have choices from a clock to a video player that rounds up TV broadcasts from around the world.

An Internet-based operating system presumes a permanent and fast Internet connection. Ironically, the U.S. is far behind other developed nations in providing this access at a reasonable price. But as network ubiquity grows, a whole new generation of users may see no distinction between the site where you share your life with your friends and the place where you do serious work.