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.
By definition, every huge success has a strong growth strategy :)
That said, it often isn't obvious from the beginning. When I met Dropbox at YC, I don't recall them having any great plans for how to grow. I think that was discovered later on.
Also, that often-repeated Craigslist claim about AirBnB is actually false. They got very few listings from Craigslist.
Well, the way you should think about it is that in order to become big successful businesses, Airbnb and Dropbox needed to have incredibly strong growth strategies. Airbnb struggled for many years before hitting a solid growth strategy (which is far beyond CL) and Dropbox tried tons of other user acquisition strategies before realizing that the double-sided reward system was their way to go.
* 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...