Take Facebook as an example. Is Facebook worth $50b, per the recent Goldman Sachs investment? It's possibly worth much more, if we assume that Facebook is the winner-take-all in a network market (social networking). Are there significant uncertainties related to Facebook's revenue model and the company's ability to fend off substitution threats? Yes. But this is consistent with the risk/reward profile that comes with an equity investment in a privately held, high-growth tech company.