Facebook comments and photographs could be held against employees, said training organisation AstroTech in a statement on Monday.
"We're seeing increasing numbers of companies coming to courses trying to navigate their way around rapid changes in information technology law or those laws that apply to it. And how Facebook is used is a real concern," said chief executive officer training company AstroTech, which offers courses in internet technology and the law.
She warned that people have been charged for posting defamatory messages about their boss, or losing their sick benefits when the company finds out that the ill person's photographs and status updates indicate they are well.
A Canadian insurer Manulife confirmed that it regularly trawled social networking sites looking for those who abused benefits.
AstroTech's course is facilitated by lawyer Warren Weertman who has helped draft legislation in South Africa.
He said: "If you are in an employer, employee relationship, not just Facebook but blogging, you have to remember to always act in the employer's best interest, that is a common law compulsion."
The Regulation of Communications Act also allows employers to intercept and monitor communications, which includes blogs, emails, telephone calls, internet activity and faxes.
Employees should not disclose company information which can be subject to the Copyright Act. This could be a breach of an employee's fiduciary duties.
"If you post: 'my boss is an idiot' or write something criticising the policies of the company you work for, you are bringing them into disrepute and that is unlawful," he said.
He advised employers to have clear policies and procedures on social network, as well as guidelines on what is not acceptable.
Some companies might encourage client relationship managers and key executives to use LinkedIn, as an example, because its focus is on business networking.
Company urges caution with Facebook - Mail & Guardian Online