One of the biggest reasons that companies are installing web-filtering software is to shut down staff access to social networking sites.

According to a survey by Email Systems, CTOs and IT administrators are increasingly choosing to specifically block access to Facebook, alongside the other chief time-sapping websites – hotmail and gmail.

More than 47 percent of of Email Systems’ web filtering customers have already blocked Facebook as an implicitly selected site and 83 percent have blocked it by category (social networking).

Youtube is also blacklisted by more than half of Email System's customers and 64 per cent block streaming and download video content from its users.

One in five companies has disciplined an employee for violating blog or message board policies in the past year, while 7.1 percent actually fired an employee for such infractions.