Because Facebook and Twitter have become as central to workplace conversation as the company cafeteria, labor officials are pushing changes regarding restrictions on employees’ speech.

Businesses often seek to discourage comments that paint them in a negative light. Don’t discuss company matters publicly, a typical social media policy will say, and don’t disparage managers, co-workers or the company itself. Violations can be a firing offense.

But in a series of recent rulings and advisories, regulators have declared many such blanket rules illegal. The National Labor Relations Board says workers have a right to discuss work conditions freely and without fear of retribution, whether the discussion takes place at the office or on Facebook.

In addition to ordering the reinstatement of various workers fired for their posts on social networks, the agency has pushed companies nationwide, including giants such as General Motors, Target and Costco, to rewrite social media guidelines.

“Many view social media as the new water cooler,” said Mark G. Pearce, the board’s chairman, noting that federal law has long protected the right of employees to discuss work-related matters. “All we’re doing is applying traditional rules to a new technology.”

The decisions come amid a broader debate over what constitutes appropriate discussion on social networks. Schools and universities are wrestling with online bullying and student disclosures about drug use.

Governments worry about what police officers and teachers say and do online on their own time. Even corporate chieftains are finding that their online comments can run afoul of securities regulators.

The labor board’s rulings, which apply to virtually all of the private sector, generally tell companies that it is illegal to adopt broad social media policies — like bans on “disrespectful” comments or posts that criticize the employer — if those policies discourage workers from exercising their right to communicate with one another with the aim of improving wages, benefits or working conditions.

But the agency has also found that it is OK for employers to act against a lone worker ranting on the Internet.

The board’s moves have upset some companies, particularly because it is taking a law enacted in the industrial era, principally to protect workers’ right to unionize, and applying it to the digital activities of nearly all workers. But board officials say they are merely adapting the provisions of the National Labor Relations Act, enacted in 1935, to the 21st century workplace.

Lewis L. Maltby, president of the National Workrights Institute, said social media rights have been looming larger in the workplace.

“No one should be fired for anything they post that’s legal, off-duty and not job-related,” Maltby said.