Whether or not you are represented by a union, federal law gives you the right to join together with coworkers to improve your lives at work - including joining together in cyberspace, such as on Facebook.
Federal law protects your right to engage in not only union activity, but also "protected concerted" activity. You have the right to address work-related issues and share information about pay, benefits, and working conditions with co-workers and with a union. You have the right to take action with one or more co-workers to improve your working conditions by, among other means, raising work-related complaints directly with your employer or with a government agency, or seeking help to form a union. Using social media can be a form of protected concerted activity. You have the right to address work-related issues and share information about pay, benefits, and working conditions with coworkers on Facebook, YouTube, and other social media. But just individually griping about some aspect of work is not "concerted activity": what you say must have some relation to group action, or seek to initiate, induce, or prepare for group action, or bring a group complaint to the attention of management. Such activity is not protected if you say things about your employer that are egregiously offensive or knowingly and deliberately false, or if you publicly disparage your employer's products or services without relating your complaints to any labor controversy.