Company valuations are messier than tulip bulb valuations though. With tulip bulb valuations you are saying that since someone will pay x the bulb is valued at x. With company valuations you are saying that since someone will buy 1/n of the company for x then the company is worth n*x. This makes sense because people really want a piece of facebook and valuations do a great job of setting that price. But the price is also used to determine how much the entire company will go for, which is how the number is often thought of.
As long as the valuation goes up and you have the opportunity to sell for a net gain then it is a good investment. DHH's thoughts here are important beacause they call attention to a valuation bubble. I think the main controversy lies in his creative use of words.