Facebook investors have increased pressure on Chairman and CEO Mark Zuckerberg to step down. This came after a New York Times investigation suggested that the social network hired a Republican-owned political consulting and PR firm that "dug up dirt on its competitors".

The attack on 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.

According to a report in The Guardian on Saturday, Jonas Kron, Senior Vice President at Trillium Asset Management which owns a substantial stake in Facebook, "called on Zuckerberg to step down as board chairman in the wake of the report".

"Facebook is behaving like it's a special snowflake. It's not. It is a company and companies need to have a separation of chair and CEO," Kron was quoted as saying.

The New York Times report suggested that Facebook hired Definers Public Affairs; a Washington, DC-based conservative firm which did PR works for the social networking giant "and dug up dirt on the company's competitors and its critics".

In a press call, Zuckerberg denied he had any prior knowledge about this firm. "After reading the article, I got on the phone with our team and we are no longer working with this firm," he said.

Another Facebook investor Natasha Lamb from Arjuna Capital said the combined role of chairman and chief executive means that "Facebook can avoid properly fixing problems inside the company", said the report. In a statement, Facebook COO Sheryl Sandberg also denied any knowledge of the firm.