According to the Human Resource Executive Online, employees are actually covered by a number of different statutes that protect their rights to say whatever they want online. For example, employers in New York cannot take any adverse action against an employee based on that employee's engagement in "legal recreational activities" if the employee engages in the activities outside of working hours, off of the employer's premises, without using the employer's property. Blogging and commenting online both fall within the "legal recreational activities" category.
Also, apparently the National Labor Relations Act (NLRA), thought by many to only cover employees who belong to unions, in fact extends to non-union employees. Rather than try to paraphrase, here is a direct quote from the Human Resource Executive Online post pertaining to NLRA:
While employers are permitted to lay out policies as to what employees may blog about in relation to work, employers cannot fashion policies that have the effect of chilling an employee's exercise of his or her Section 7 rights, nor can employers discipline employees for blogging about "wages, hours, or terms or conditions of employment," such as the company's pay scale or vacation policy. See Timekeeping Sys., Inc., 323 N.L.R.B. 244 (1997).
Does this mean you should feel free to blog about how you hate your job or your boss or to add those lingerie shots to your Facebook profile? No. However, it does mean that if your company wants you to hand over your social media account passwords or forbids you to blog, you might want to polish up your resume.
Sort of related but not really: how much do Dooce's former employers regret firing her now?
(Oh, and even though it goes without saying: I'm not a lawyer so don't take my word for any of this; I'm just passing on info I found interesting.)