Not to marginalize these guy's success in any way, but being part of the winter 09 ycombinator class probably helped in a big way (right around time of their major bump)
The YC pedigree probably helped with capital and hiring, but I'm curious how they solved the chicken and egg problem of getting enough rentals to attract enough renters to attract more rentals... etc. Alot of hard work and selling, I imagine.
Well don't discount the PR and hype. Reputation plays a huge factor on a service like this. Yes, hard work and persistence is the real factor of this success.