London's third great function, since the seventeenth century, has been that of national and international bourse: the exchange of stocks and shares, banking, commerce and, increasingly, insurance.

Disastrous was the way one trader on the floor of the New York Stock Exchange described the drop in US shares on Thursday.

According to SharesPost, an exchange for shares of private companies, Facebook is worth more than $45 billion. Google's market value is more than four times that amount.