As Facebook and Twitter become as central to workplace conversation as the company cafeteria, federal regulators are ordering employers to scale back policies that limit what workers can say online. While employers often seek to discourage comments that paint them in a negative light. a series of recent rulings and advisories declare many such blanket restrictions 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 like General Motors, Target and Costco, to rewrite their social media rules.