Extrapolating Facebook's true worth from Microsoft's investment is a ridiculous exercise ... The investment ... was a price Microsoft had to pay to nail down the partnership.This was $240M to gain share for Microsoft's advertising platform at the expense of Google and Yahoo. It says nothing about the market value of Facebook.
Partnering ... is far more about gaining future strategic options and blocking the advance of ... Google ... than about making a financial gain.
Update: Mike Masnick reports on rumors that some crazy hedge funds might have gone in at the same $15B valuation for Facebook. As Mike says, "Those hedge funds don't get any of those additional benefits that Microsoft gets." If true, this news would support a $15B valuation for Facebook, at least if you believe those hedge funds are responsible stewards of their investors' money.
Update: John Battelle says, "I think no one ... has truly grokked what Facebook has a shot at doing - Adsense driven not by search queries, but by personal profile." Call me a skeptic on this one. I think personalized advertising will produce much higher returns by reaching back to the last action with purchase intent than by targeting a coarse personal profile.