Facebook investors have called on the company’s chief executive Mark Zuckerberg to step down as chairman, following reports that the company hired a public relations firm to smear its critics by drawing links to George Soros. The attack on Mr Zuckerberg is set to complicate the daunting challenge facing Sir Nick Clegg, Facebook's new global head of policy and communications, who joined last month and has been asked to conduct a review of Facebook's use of lobbying firms. Jonas Kron, a senior vice president at Trillium Asset Management, a US investor which owns an £8.5m stake in Facebook, last night...