The National Labor Relations Board affirmed a previous ruling, and said that a company that fired workers for discussing working conditions on Facebook was wrong. The Board said that the discussion was among co-workers, and thus falls under the category of protected concerted activity under the National Labor Relations Act. The NLRB says that they have two dozen cases relating to worker complaints on social media sites.
See "NLRB Faults Company for Firing Workers Over Facebook Posts", Melanie Trottman, The Wall Street Journal, May 18, 2011