While I agree about the P/E ratios, I think Facebook has definitely started monetizing it's users, short of charging them a monthly fee anyway. Logging in now gives me a home page with banner ads to the left, and at least 3-4 ads in my 'News Feed'. Not to mention the marketplace aspect of it. If Facebook becomes the de facto identity manager on the web, I could see a sky high valuation like this, but I think they will piss off their users if they keep going like this.
I read somewhere else that they suspect that Microsoft just wants the 5% so it has more control over serving ads, much like Google owns 5% of AOL so it can serve a ton of ads to AOL users/visitors.
The ads on Facebook get abysmal clickthrough rates (often less than 0.1%). $10 billion is a P/E of more than 300, and even for a very hot company, that is pretty far into outer space. If these are in fact the numbers being thrown around behind the scenes, you're probably right that it's because Microsoft wants a place to improve its ad serving (and to deny Google that same place).
I read somewhere else that they suspect that Microsoft just wants the 5% so it has more control over serving ads, much like Google owns 5% of AOL so it can serve a ton of ads to AOL users/visitors.