What makes a successful car-sharing scheme?

On paper, car-sharing is a simple business model, acting essentially as an on-demand version of traditional vehicle rental. But, in practice, even some of the biggest names in the automotive industry—most notably General Motors—have failed to make it work. These players could be excused given that shared mobility, though increasing in importance, is not their core focus. However, the failure of dedicated, and initially successful, car-sharing company Autolib shows that even with a sole focus, car-sharing is a tricky market to master. So how could future entrants thrive in this space?