If an employee posts on social media that a company is bad to work for, can they get fired or reprimanded for it? What if they gave specific examples, like how the employer reduced their wage without an explanation?
You can be fired in Canada for criticizing the employer, or even complaining about the weather. There is a distinction between Termination Without Cause and Termination With Cause. In the latter case, which requires a serious reason related to the employee's conduct, you can be fired without advance notice and with no severance pay. If the employees actions are fundamentally inconsistent with their obligation to the employer or are substantially prejudicial to the business in a way that damages employer's business or reputation, they may be terminated with cause. Examples of cause would include insubordination, theft, or abusing customers.
If you want to fire a person without cause (and assuming that this is an indefinite employment contract as opposed to a fixed-term contract), you have to give "reasonable notice". There is a statutory minimum, but the courts usually apply a higher common law standard which means that you need to hire a labor lawyer to know what that period is, though 24 months is apparently a relatively safe figure (not always safe). The factors entering into that decision are described here (kind of job, length of service, age, availability of similar jobs; plus, how the termination was handled). You may also owe severance or termination pay, related to length of service and wages. Here is a calculator for Ontario.
This article covers some instances in Québec where social media criticism did result in successful suspension, indicating that the employee's duty of loyalty is not entirely null when it comes to social media. In the BC case of Kim v. ITU, the court found that the dismissal over social media posts critical of the company was not for cause. However, part of the company's failure in this case was that they failed to respond immediately to what they saw as inappropriate behavior (boorish Twitter behavior).
Assuming that the statements made are accurate and expressed respectfully, the prospects for Termination With Cause are significantly diminished. The prospects for some disciplinary action (suspension for a period of time) remains high -- multiple terminations were modified to long suspensions.