NEXT, I scoured its listings. There was a luxury apartment in north London owned by an information technology professional who rents it out for £65 a night when he’s out of town. In Bloomsbury, an American social worker had a room for £35 a night. But my dash across Europe necessitated a bed near St. Pancras International train station, so I opted for a “clean simple room with a chrome silver double bed” in the once-seedy district of Kings Cross for £50, owned by a 39-year-old vintage-clothing buyer.

Once you’ve found your room on Crashpadder, you can interact with the host through the internal messaging system and ask any questions you might have. (Do I need to bring towels? Do you have cats?) Hosts can ask for the money either up front or upon arrival.

A week after booking my room, I rolled my bag through the back streets of Kings Cross, aiming for a 1920s red-brick council building, where my host, Rachelle Hungerford-Boyle, lives on the third floor. It was the kind of unspectacular housing block that makes up vast swaths of the city, scattered in between the genteel stucco mansions that make it into the films.

I was buzzed through an electronic gate, and found Ms. Hungerford-Boyle hanging over the balcony, beckoning me up. I maneuvered my way through a soccer game among 8-year-olds and marched up to meet her. Inside her small apartment, the living room was dotted with extravagant secondhand dresses she had picked up in markets around the world. She’d sell her prize possession, a fluffy ball gown dangling from the window, only to Lady Gaga, she said.

My room, just over 10 by 10 feet, with a double bed and a door that locked, was good enough as a base. There was also a kitchen I was free to use, and a bathroom and shower. As this was my first social B&B experience, it was also immediately clear what I wasn’t getting. I couldn’t walk around naked, for example, and there wasn’t a concierge who could make a dinner reservation for me. But there was Ms. Hungerford-Boyle, who, while making her temporary bed in the living room (where she sleeps when she has guests), told me how quickly the area was gentrifying, and suggested I visit Simmons, a trendy bar around the corner.

At Simmons later that evening, I thought about the £50 I had paid for Ms. Hungerford-Boyle’s spare room. Founded in 2008, Crashpadder hasn’t expanded as much as AirBnB (in early July it had listings in 898 cities compared with AirBnB’s 5,378). I had a feeling that prices were somewhat scattershot, freed from the self-regulating bonds of a more mature marketplace. At my next stop, I wanted more.

The next morning I caught a Eurostar train to Paris, where my social B&B was booked through AirBnB.com. The site, which is based in San Francisco, was started by Joe Gebbia and Brian Chesky, roommates who decided to rent out their spare bedroom to people visiting the city for a design conference. An idea was born.