Dissenting Justice says the USDA is a better target than Breitbart for Sherrod's suit:

In order to prevail, she will have to prove that Breitbart acted with actual malice. This standard requires that Sherrod prove that Breitbart published information about her that he knew was false or that he acted with reckless disregard for the truth. Although this is normally a tough standard, Sherrod might prevail, given the circumstances of the scandal. It will be difficult, however, given the First Amendment interests at stake. 

Sherrod probably has an even stronger lawsuit against the government. USDA officials forced her out of her government job without due process of law. She was not given the opportunity to explain herself, nor did her supervisors listen to the full speech before demanding her resignation.