AirBnB's big secret was siphoning off Craigslist traffic. It really was a bit of marketing genius (note that they sold boxes of cereal all across the country for $50 each using Craigslist; they were experienced Craigslist spammers.)

If you had a rental property up on CL, you got an email asking you to list it. They would often times relist that property on CL with a link back to AirBnB. They had a better product than CL for rental properties, so it worked like a charm.