Are social media posts protected under the First Amendment?

Employers may prohibit social networking on company time or posting anything negative about the company. Therefore, if your employor had a social networking policy and you violated it, that could be grounds for disciplinary action or posibly even dismissal.
Chances are you're an "at will" employee. This means that your employer can place you on administrative leave as it deems fit, so long as it does not violate any employment contract or union agreement that exists, or if doing so constitutes some form of actionable discrimination. Accordingly, you should have refrained from engaging in such activity. 
It should be noted that while an employer has the right to take some form of action against an employee regarding prohibited uses of social media, many states have laws which prohibit employers from disciplining employees from certain conduct outside of the workplace and many are written broadly enough to cover online activity and any disparaging remarks that an employee makes. Additionally, a social networking policy that is too restrictive may be a violation of an employee&rsquos rights under the National Labor Relations Act NLRA. The Act protects an employee's rights to participate in &ldquoconcerted activities for the purpose of collective bargaining or other mutual aid or protection". Accordingly, employees have the right to speak out in an effort to improve workplace conditions. Consequently, social networking policies that prohibit such online activity that might be considered a violation an employee&rsquos rights.
At this point, you can consult directly with an employment law attorney they can best advise you of your rights under the circumstances.