Lyft, Uber, Pinterest: Are internet unicorns really worth billions?

This week has been a momentous one for the US stock market. Lyft, the ride-hailing company, sold its shares to the public for the first time, heralding a march of the "unicorns". A stream of these businesses - which are defined as private, venture capital-backed firms worth over $1bn - are set to follow, including Lyft's rival Uber, online scrapbook company Pinterest and home-sharing site AirBnB. And they are attracting some staggering valuations. Uber, for example, could be worth as much as $120bn when it floats.