Microsoft's $240 million investment in Facebook is being interpreted (by the AP, among others) as placing a $15 billion value on the whole operation.
This is silly for a number of fairly obvious reasons. Perhaps the most obvious is that Microsoft isn't trying to buy Facebook, but rather has other objectives in mind.
Anyone who has watched Microsoft for any amount of time knows the pattern: Partner, learn, copy, crush.
For an amount that (for Microsoft) is essentially chump change, it gets an inside view into a process that it desperately needs to understand so that it can adapt its other business processes to the new realities introduced by social networking.
This is a completely different play than Rupert Murdoch's purchase of Myspace, and in no way does it project to a $15 billion valuation of Facebook as a whole.