One of the reasons for why Microsoft's $240M investment for 1.6 percent of Facebook does *not* necessarily imply that Facebook is worth $15B. Microsoft very likely got some "preferred" treatment in this deal.

Quoted: Microsoft is probably getting some kind of option for their $240 million…. We don't know what it is, but it's probably the right to purchase shares of Facebook at some known price in the future. The value of that option is very hard for business journalists to calculate, but it's NOT hard for Microsoft's CFO to calculate, and it's probably very significant. You have to deduct the value of the option from the purchase price before you can multiply and get the valuation. Almost certainly, the $15 billion valuation is ridiculous.

The other reason (curiously omitted) is that Microsoft received exclusive rights to international advertising on Facebook. This also has to be deducted from the purchase price.