Has everybody missed the point?
I would have thought that it was plainly obvious that the company isn't actually worth $15bn. I am fairly certain Facebook know that as well.
Microsoft: We want to control all of your advertising and are willing to pay for it.
Facebook: I am sure we can work something out. How about you buy 1.6% of the company for $240m?
Microsoft: Seems a little expensive. Are you sure you are worth $15bn?
Facebook: No. But you will make your money back over the next 4 years of your advertising contract and we will look like we are worth $15bn.
Microsoft: So if all goes to plan we end up with a chunk of you for free which is essentially our profits and you get to IPO yourself valuing the company at a ridiculously over-inflated $15bn+ before you dump your stocks just before everybody realises its not worth that much.
Facebook: Exactly.
Microsoft: Sounds good. We're in.
Seriously though it doesn't take more than some pretty simple maths to work out that unless they find some extra revenue streams other than advertising there is absolutely no way they can be worth $15bn (or even close to).
Don't get me wrong, I like facebook and probably sign on every other day. Its a great way to share photos and keep in contact with long lost friends and is certainly much easier than friends reunited ever was. But the advertising is starting to get pretty intrusive and I can only assume that with this deal it will only get more so. I don't believe it is a fad, but I think it needs to grow up out of its MySpace ideals. For example it could be a fantastic resource for single sign-on to lots of different sites, assuming they don't try and charge site for the privilege of course.