Square's IPO: the beginning of the end of the unicorn-driven tech bubble?

The $1bn-plus valuation of a number of Silicon Valley firms far outstrips their ability to deliver such returns - and many investors now believe it's a question of when, not if, the bubble will burst

There are a lot of people in Silicon Valley and New York betting on "unicorns" - the new breed of tech startups like Uber, Square, Airbnb and Snapchat that have been valued at least $1bn.

Not since the last dotcom boom has so much money been poured into so many hyped companies. As startup founders and their investors hope to turn their paper unicorn fortunes into cold hard cash, some of Silicon Valley's most successful investors are warning a reckoning is coming. And on Monday that magical thinking faces one of its biggest tests.