Rob Coneybeer doesn’t often drive the silver BMW Z3 parked near his house. Rather, he purchased the convertible primarily for other people to use. “I bought the car to list it on RelayRides ,” says the Shasta Ventures managing director, referring to the peer-to-peer car-sharing platform that has received $13 million in funding from Shasta, Google Ventures, and GM Ventures, among others. For $15 an hour, anyone in San Francisco can rent Coneybeer’s wheels. This arrangement, he believes, represents the future of car ownership. “You know in Austin Powers, when Dr. Evil says ‘$100 billion’ and puts his pinkie to his mouth?” says Coneybeer. “Car sharing is a billion-dollar opportunity.”

A lot of sharing companies weren’t providing strong value propositions, but got caught up in the hype,” says Reinhart.

In the two years since I covered the “sharing economy” for Fast Company (“spawned by a confluence of the economic crisis, environmental concerns, and the maturation of the social web, an entirely new generation of businesses is popping up,” I wrote), dozens of platforms have emerged to facilitate the selling, swapping, and renting of every conceivable asset. The success of Airbnb, which reportedly has a valuation of $2.5 billion, has spurred entrepreneurs to apply the model to everything from pasta makers to unused garage space.

But when it comes to business models, we’re learning that Airbnb may be more the exception. Tomasz Tunguz, a VC at Redpoint Ventures, says a platform’s viability depends on the value of listed items, the profit an owner can make, and the cut for the intermediary. Home rental is one of the few areas where there’s payoff for both owners and renters. After that, the math becomes dicey. Says James Reinhart, CEO of ThredUp, a service for reselling clothes, “A lot of sharing companies weren’t providing strong value propositions, but got caught up in the hype.”

This has played out most starkly in the neighborhood sharing space, where some 10 startups fell prey to the idea that neighbors were the optimal sharing group. “Developing an alternative to Craigslist has proven difficult,” says Rachel Botsman, coauthor of What’s Mine Is Yours: The Rise of Collaborative Consumption. “People assumed that location is the primary factor. It’s not.” If the effort required to pick up the good is the same as going to the store and the savings aren’t significant, the motivation won’t be there for a marketplace to work.

Even car-sharing outfits are facing the challenges of scaling. While people point to the $491 million sale of Zipcar to Avis as proof the model works (despite Zipcar being an owned fleet, not P2P), Reinhart has a more sober read: “It took Zipcar more than a decade to build that business, and they were barely profitable. My sense is that this was not because of the economics of owning the cars but because of [low] utilization.” Craig Shapiro, founder of the Collaborative Fund, which has invested in more than 20 sharing startups, says the gap between intent and reality is another inhibitor: “If you tell 10 people about car sharing, 7 will say, ‘Sounds neat.’ But when you say, ‘Why don’t you post your car on RelayRides?’ they say, ‘What if someone spills coffee?’ And it drops to 1 or 2.”

But an opportunity clearly exists for companies that are able to provide real value and an unrivaled experience. “The first generation facilitated a transaction,” says Shapiro. “Newer startups prolong their involvement.” DogVacay–which is targeting the estimated $8 billion pet services and boarding market by connecting traveling dog owners with temporary hosts–provides $4 million in liability coverage, has developed an app so hosts can share pictures of the dog with its owner, and is exploring providing GPS collars so pups can be tracked. “I no longer think of us as a peer-to-peer marketplace,” says DogVacay founder Aaron Hirschhorn. “I consider it a service where we provide the highest quality possible.”

How three companies are adapting to the modern sharing landscape.