http://www.forbes.com/sites/jonathan...ers-valuation/
This article indicates that private investors are the ones being rooked into thinking it's worth 40 billion. Most telling is this quote:
"...the people who stand to make the most money are the ones providing the public estimates of its value."
In other words, it's valued that way because everyone agrees they can make a lot of money if it's valued that way. Cozy.