I won't argue that growth is not why they have that valuation. But, Google and Facebook, when faced with growth problems did not say, "How can we outsource our data growth problems?" They used it as a competitive advantage and a way to raise the barrier to entry for competitors. If Dropbox is paying 25%-50% more for storage than a competitor, they're going to lose as soon as a competitor comes along that gets all the other bits right.

All other things being equal, the company with lower costs wins. I'm not suggesting any other company is currently equal to Dropbox (it's what I use, and I'm constantly surprised by the number of my non-nerd friends who have the Dropbox icon on their desktop)...but, there are an awful lot of smart people out there who would love to eat their lunch.