WASHINGTON (AP) — In the age of instant tweets and impulsive Facebook posts, some companies are still trying to figure out how they can limit what their employees say about work online without running afoul of the law.

Confusion about what workers can or can't post has led to a surge of more than 100 complaints at the National Labor Relations Board — most within the past year — and created uncertainty for businesses about how far their social media policies can go.

"Employers are struggling to figure out what the right policies are and what they should do when these cases arise," said Michael Eastman, labor law policy director at the U.S. Chamber of Commerce.

In one case, a Chicago-area car salesman was fired after going on Facebook to complain that his BMW dealership served overcooked hot dogs, stale buns and other cheap food instead of nicer fare at an event to roll out a posh new car model.

The NLRB's enforcement office found the comments were legally protected because the salesman was expressing concerns about the terms and conditions of his job, frustrations he had earlier shared in person with other employees.

But the board's attorneys reached the opposite conclusion in the case of a Wal-Mart employee who went on Facebook to complain about management "tyranny" and used an off-color Spanish word to refer to a female assistant manager. The worker was suspended for one day and disqualified from seeking promotion for a year.

The board said the postings were "an individual gripe" rather than an effort to discuss work conditions with co-workers and declined to take action against the retailer.

Those cases are among 14 investigations the board's acting general counsel, Lafe Solomon, discussed in a lengthy report last month on the rise in social media cases. Solomon says federal law permits employees to talk with co-workers about their jobs and working conditions without reprisal — whether that conversation takes place around the water cooler or on Facebook or Twitter.

"Most of the social media policies that we've been presented are very, very overbroad," Solomon said in an interview. "They say you can't disparage or criticize the company in any way on social media, and that is not true under the law."

The number of cases spiked last year after the board sided with a Connecticut woman fired from an ambulance company after she went on Facebook to criticize her boss. That case settled earlier this year, with the company agreeing to change its blogging and Internet policy that had banned workers from discussing the company over the Internet.

The National Labor Relations Act protects both union and nonunion workers when they engage in "protected concerted activity" — coming together to discuss working conditions. But when online comments might be seen by hundreds or thousands of eyeballs, companies are concerned about the effect of disparaging remarks.

Doreen Davis, a management-side labor lawyer based in Philadelphia, said many of her corporate clients are often "surprised and upset" when they learn they can't simply terminate employees for talking about work online.

"All of us on the management side are being inundated with calls and inquiries from clients about this," Davis said. "A lot of companies want their social media policies reviewed or they want to establish one for the first time."

But the NLRB's Solomon also warns workers that not everything they write on Facebook or Twitter will be permissible under the law just because it discusses their job.

"A lot of Facebook, by its very nature, starts out as mere griping," Solomon said. "We need some evidence either before, during or after that you are looking to your fellow employees to engage in some sort of group action."

In one case, an employee at an Indiana emergency transportation and fire protection company was fired after writing on the Facebook wall of her U.S. senator, Republican Dick Lugar, to complain that her company skimped on wages and that its cheap service compromised the quality of care.

The NLRB's enforcement office declined to take up her case, saying that the employee didn't discuss her complaints with other workers or show any attempt to take employee complaints to management. She may have been trying to make a public official aware of problems with emergency medical services in Indiana, but board attorneys said that wasn't enough to protect her under the law.

While there are more than 100 cases pending before the board, only one has actually led to a formal ruling. Earlier this month, an administrative law judge at the agency found that a Buffalo, N.Y., nonprofit group illegally fired five workers after they posted Facebook comments complaining about workload and staffing issues.

The judge ordered the group, Hispanics United of Buffalo, to reinstate the five employees and award them back pay.

The Chamber of Commerce's Eastman said it's too early to criticize how the board is interpreting the law, but he wants to see what happens in closer cases where an employee goes "over the top" with criticism of a supervisor of employer.

"Where will the board draw the line between concerted activity and an employer's legitimate non-disparagement policy?" Eastman said.