San Francisco, Dec 6 (IANS): Facebook's board of directors has defended Chief Operating Officer (COO) Sheryl Sandberg's request to look into billionaire critic George Soros' alleged interest in tarnishing the image of the social networking giant.

According to a report in CNET on Wednesday, the board said in a letter that Sandberg "asked her staff to look into Soros' financial motivations after he called companies like Facebook and Google a "menace".

"Facebook staff immediately initiated research to attempt to understand the motivations driving the criticism, financial or otherwise," read the letter.

"Sandberg's question was entirely appropriate given her role as COO," it added.

A New York Times investigation in November suggested that the social network hired a Republican-owned political consulting and PR firm that "dug up dirt on its competitors" including Soros.

Facebook hired Definers Public Affairs, a Washington DC-based conservative firm which did PR work for the social networking giant "and dug up dirt on the company's competitors and its critics", said the report.

Reacting to the report, Facebook CEO Zuckerberg and Sandberg denied they had any prior knowledge about this firm.