According to research by security company Sophos, 43 percent of workers polled said their employer blocks Facebook access completely.
A further 7 percent said access is restricted depending on whether it's required for a particular job.
"I think it's a growing concern for employers for a number of reasons," said Graham Cluley, senior technology consultant at Sophos. "The most pressing concern at the moment is one of productivity...Some people are spending an inordinate amount of time on nonwork-related Web sites."
Cluley added it is difficult to tell when people are using a social-networking site when they are sitting at a computer.
The issue of security was also raised by the Sophos research. In a separate poll by the company, 66 percent of workers said they are concerned about colleagues sharing information on Facebook.
Details such as employment history and mobile phone numbers have been found on the site and could be used for identity theft or to launch corporate phishing attacks, security experts warn.
Sophos research found that 41 percent of Facebook users are willing to
Sophos last week released the results of a Facebook ID probe indicating that a relatively large percentage of people were willing to divulge e-mail addresses, dates of birth, phone numbers and other data to a stranger--a fake character created by Sophos, in this case--who requested "friend" status of 200 randomly selected Facebook members.
"Everyone's just sort of letting it all hang out online without thinking who might be watching," Cluley said.
Facebook noted that it provides users with theand "welcomes every opportunity to educate users about how to protect their data online."
Of the 50 percent of companies that allow access to Facebook, Cluley said, some view it as a valuable networking tool while others are nervous about the possibility of an employee backlash to a ban.
Tim Ferguson of Silicon.com reported from London.