A few employees at our company have started posting criticisms of the company on Facebook. A manager is Facebook friends with one of them, and he showed me some of the comments (I'm the human resources director). These employees are snarking about everything from how much they are paid to how small their cubicles are and how "last year" their coworker's outfits are. They also mentioned doing some kind of "anonymous" letter to the company's owner, asking for annual cost-of-living raises and additional bonuses based on performance. I am really concerned that their poor attitudes will start to affect the morale of their team and other employees. Can I discipline them for posting these kinds of comments?
Don't do it! Disciplining employees for online posts that criticize their working terms and conditions -- including their pay and their cubicles, though not necessarily their coworker's fashion sense -- could violate the National Labor Relations Act (NLRA).
The NLRA gives employees the right to communicate with each other about the terms and conditions of their employment, and to join together in bringing these concerns to the employer. This is called "protected concerted activity," and employers may not punish employees for engaging in these communications, whether or not the employees are represented by a union.
The employees don't have to be discussing a union campaign or expressly invoking their rights to act collectively. It is enough that two or more employees are engaged in a conversation about their job conditions, particularly if part of the discussion is about raising those concerns with their employer. That's what you've got on your hands: a group of employees discussing pay and how to get more of it from their employer. That they are also criticizing a coworker and being snarky doesn't make their conversation any less protected.
In the past few years, the National Labor Relations Board (NLRB) has been extremely active in going after employers who fire or discipline employees for posting critical comments about the company on social media sites or blogs. Employers that run non-union workplaces have been surprised by some of these enforcement actions. Typically, non-union companies don't pay much attention to labor laws. However, these laws protect not just workers in a union; they protect all employees when they are acting collectively (including by communicating with each other) to improve their working conditions.