Something's only worth what somebody else is willing to pay for it. Try buying the NYT for $950 million one share at a time. You will actually drive the price up, since there's displayed demand. The market cap is just the sum of all of its shares multiplied by the last price the share traded at. It would take considerably more to purchase the entirety of the company.

That being said, Instagram sold for an outrageous amount. But this article is rather naive in how stock markets work or how companies are valued.