"Five years ago, in the summer of 2010, roughly 47,000 people stayed with Airbnb hosts. This summer, nearly 17 million total guests stayed with Airbnb hosts around the world. That means that in the last five years, summer travel on Airbnb has grown 353 times over."
Source: Airbnb's Summer Report, September 2015
Note - 'Summer' means 'end of May to early September'