I’m one of those people who thought Airbnb would never work. I thought people would never rent out space in their homes to near strangers. But I was clearly wrong. Eleven million travelers have stayed in Airbnb destinations, according to data shared by the company. Roughly 550,000 homes are now being shared by hosts. Airbnb is more popular in Europe than it is even in the United States. Paris is the largest destination city.

And Airbnb is only a piece of the peer-to-peer economy. People are renting out their cars to people they don’t know, dropping off their pets with people they don’t know, renting power tools to people they don’t know.

In retrospect, I underestimated the power of a few trends that make the peer-to-peer economy possible. First, I underestimated the effects of middle-class stagnation. With wages flat and families squeezed, many people have to return to the boardinghouse model of yesteryear. They have to rent out rooms to cover their mortgage or rent.

Second, I underestimated the power that liberal arts majors would have on the economy. Millions of people have finished college with a hunger for travel and local contact, but without much money. They would rather stay in spare rooms in residential neighborhoods than in homogenized hotels in commercial areas, especially if they get to have breakfast with the hosts in the morning.