Didier Sornette — French Scientist born on June 25, 1957,

Didier Sornette is Professor on the Chair of Entrepreneurial Risks at the Swiss Federal Institute of Technology Zurich. He is also a professor of the Swiss Finance Institute, and a professor associated with both the department of Physics and the department of Earth Sciences at ETH Zurich. He was previously jointly a Professor of Geophysics at UCLA, Los Angeles California and a Research Director on the theory and prediction of complex systems at the French National Centre for Scientific Research... (wikipedia)

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.