What strikes me about Dropbox and Airbnb more than anything is that they had incredibly strong growth strategies.

* Dropbox used rewards for more storage, as well as its fundamental file-sharing tools, to encourage users to sign up their friends and family.

* Airbnb gamed Craigslist [1] to encourage property owners to signup on Airbnb. With a strong supply of rental properties, Airbnb was able to build a true case for its value in the minds of travelers.

[1] http://venturebeat.com/2011/06/02/airbnb-admits-gaming-craig...