UK businesses are losing £6.5bn a year in lost productivity and the cost of extra bandwidth because of employees checking sites like Facebook and Bebo.
These were the findings of a survey by Global Secure Systems (GSS) in conjunction with Infosecurity Europe 2008.
It found that office workers spend at least 30 minutes a day on social networking sites, which equates to around three weeks a year.
David Hobson, managing director of GSS, told VNU, however, that these sites have, in some cases, become part of how people work and so can't simply be banned.
"Social networking sites are now integral to the way that many of the latest and youngest recruits into the workforce communicate and work."
"So, for some sectors, social networking sites may have a part to play in terms of competitive advantage or for research or as a marketing tool."
However, these sites bring with them problems.
As well as employee time wasted, they also take up bandwidth.
The survey suggest that between 15 and 20% of current bandwidth is taken up with social networking sites, and that the best move for many companies is to ban these sites altogether.
"It would appear that most chief information security officers and IT directors loathe social networking sites and would ban them completely if they had their way", said Claire Sellick, event director for Infosecurity Europe, in an interview with VNU.
"But what is also coming across is that HR departments actually welcome the use of these sites, so there is a lot of internal pushing and shoving between HR and IT over how best to manage these sites."
Hobson added that if companies do go down the route of a ban, employees will simply find something else to do to waste time.
"It comes down to a fine balancing act, and mostly a case of introducing a 'reasonable use' policy", he concluded.