At an estimated $900 million last year, Airbnb’s revenues are dwarfed by those of a Cargill. But Airbnb makes up for that in disruptive impact. Hatched out of a San Francisco apartment in 2007, it’s now the largest provider of accommodations in the world. Hotel chains such as Accor are reacting, starting to acquire their own home-sharing platforms and investing in urban timeshares. Airbnb faces regulatory resistance in markets where it runs afoul of local laws. But the company has positioned itself as a way to help the middle class, and its most effective champions are the 80 million people who have used the service as hosts or guests.