According to a John P. Quirke article in Security Management (http://www.securitymanagement.com/news/social-media-and-workplace-009636), "As companies wake up to the incredible marketing power of social media, they are also running headlong into a variety of legal concerns related to just how much power they have—or don’t have—over what their employees post on social media sites."
Quirke's article points out both the benefits - mostly marketing - and dangers of "social media." He cited, as one of the negatives, the infamous pizza video that sowed the pizza outlet in a light that proved to be untrue.
Part of the problem with "social media" is that the medium is new and relevant laws are relatively undefined; "social media" is a developing area of interest.
Quirke looks at the impact of the federal Stored Communications Act of 1986 and its impact on cases to this point, and cites examples to back up his position.
He also considers actions under the auspices of the National Labor Relations Board (NLRB).
Quirke's "bottom line" is that organizations must be "proactive in setting clear policies regarding the Internet, including employee use of social media. By setting clear guidelines, companies can protect their own interests and give employees a much-needed roadmap for appropriate conduct." He then proceeds to outline several steps organizations must undertake; several are risk management "standards," including development of policies and procedures and employee education.