Bosses can get mad when staffers vent on social media about their jobs, but they may not be able to get even. When one of Bert Martinez’ employees posted gripes about her job and the boss on Facebook last year, the publicist consulted his lawyer, who said the staffer couldn’t be fired. “The first lesson I learned is, employees are allowed to vent,” says Martinez, owner of Bert Martinez Communications in Phoenix. “If they're saying, ‘hey, it’s hard working here and I find this environment unpleasant’, you can’t fire them for that.” The employee quit a week after Martinez learned...