The biggest hotel in the world is not Hilton Worldwide (HLT) or Marriot International. It’s not Hyatt Hotels (H) nor is it Choice Hotels International (CHH) or Holiday Inn. In fact, the largest hotel in the world isn’t even a hotel… it’s Airbnb, the start-up online facilitator of short-term rentals properties. And as you might be surprised to learn, the company, which started in 2007, is valued at more than $25 billion today. Despite, or rather because of, its rocket launch-like growth, it has become a center-point in the ongoing debate over the “sharing” economy.