Even after reading this, and other similar articles, I still don't understand the current valuation models. In this case, LinkedIn makes revenue of $80-100m, b

Even after reading this, and other similar articles, I still don't understand the current valuation models. In this case, LinkedIn makes revenue of $80-100m, but what is the profit? Are such valuations solely based on projected revenue? If so, what about money pits like YouTube that operate on similar models but still yet to make profit even after a few years already. Thoughts on this always appreciated.