Economic theory dictates that the value of a company is basically the present value of its future profits. To estimate Facebook's value through its future profits, we need to have a view on its user growth and how this will evolve in the next 10 to 50 years.
When herding behaviour among investors ramps up, a stock's or index's growth rate can increase faster than exponentially, leading to more herding. This positive feedback brings the system to a tipping point. About two-thirds of the time, a crash results.