Half of all firms don't let staff visit social networks such as Facebook and Twitter from a company-owned PC or laptop, says Webroot.

Research by the security firm revealed 81 percent of companies claim to have an employee internet policy that restricts which sites can be accessed. Of these, more than two in five (42 percent) have implemented the policy as a result of an employee's inappropriate use of a social networking site and 31 percent use web-security software to monitor internet use.

Nearly four in ten (39 percent) of firm black access to Facebook, while 30 percent stop staff accessing Twitter and 27 percent prevent employees viewing video-sharing sites like YouTube.

However, 21 percent of firms allow employees to visit social networks during their lunch hour or other specific times, such as before the working day begins and 16 percent give departments such as marketing the ability to use social networks for activity relating to the company.

Over a half (53 percent) of firms are concerned about malware infections via social networks and 42 percent worry about data leakages caused by social network use, with 12 percent admitting sensitive company info has been leaked via this channel. .

"Clearly the potential impact of social networks as a threat vector has hit home for IT administrators," said Gerhard Eschelbeck, CTO of Webroot.

"Every company needs to develop a policy for social networking use and should also deploy reliable Web security services for ongoing protection against zero-day threats."

See also: 41% of web users share online passwords