When Airbnb and Uber burst on the scene a few years ago, the business world was abuzz with the potential for a new "sharing economy."

Advocates explained how these services would provide people with new ways of making money by utilizing their homes and vehicles to their full potential, while offering a service that could save energy, reduce waste and bring communities together by sharing their resources and time.

Today more than 100 companies are considered part of this industry built on digitally enabled collaboration. They range from the well-known urban transportation and accommodation services to start up communities specializing in tools and toy sharing. Even big corporations like Marriott, Walgreens and Avis have got into the sharing economy game.

Yet, as the sector has developed, the sustainability utopia of the sharing economy has come into question. Both Airbnb and Uber have had to counter claims that their services don't protect the wellbeing of the sharers who use their platforms. Uber, in particular, has been accused of all manner of unsporting business practices. On an environmental level, there are serious questions for all the transportation companies in this sector about whether their services are actually reducing the number of cars on the road-or just adding to the congestion by encouraging more people to drive for a living.

Then there is the larger issue of whether companies that have few employees but depend on a great deal of freelance labor can really be sustainable for society. Back in February, former U.S. Secretary of Labor Robert Reich lambasted the sharing economy as the "sharing the scraps economy." He argued that this new culture of on-demand work "is a reversion to the piece work of the nineteenth century-when workers had no power and no legal rights, took all the risks, and worked all hours for almost nothing." Despite these concerns, it's clear that the sharing economy could help business and society deliver on ambitious sustainability goals.

In Sustainly's new Trend Briefing, we consider 10 interesting Sharing Economy companies and the services they provide that have sustainability potential. In the transport sector we look at Lyft's "Line" service, which, like UberPool, is a ridesharing service (connected by the app) where riders are picked up along routes pre-determined by the driver. One recent MIT study found that 95% of taxi rides within New York City could be shared, but aren't-because taxi services make more by carrying lone passengers. However, the study also found that UberPool has the potential to reduce journey times by 30% as well as reduce congestion and traffic pollution. That's a potential sustainability victory for both Lyft and Uber. (Perhaps the one thing they could agree on.)

In the travel and tourism sector, we salivate with EatWith, a culinary service that matches travellers looking for good food and local culture with trained chefs and talented home cooks in cities around the world. In consumer goods, we admire the sustainable building blocks behind a company like Pley, which, through a membership scheme, offers sharing of Lego sets for families across the U.S. And we look at how the down-to-earth sharing economy is getting a fashion makeover courtesy of the tie and menswear accessory company, Fresh Neck.

As the sharing economy develops, it will find even more creative ways to make the best use of underutilized hardware while offering greater opportunities for freelance and contract work. Yet as this sector scales, the sustainability challenges-environmental and social-will increase. The truly innovative ones will find a way to maximize their lean growth while still demonstrating responsible corporate citizenship. In this way, they will show that the sharing economy can also be caring.