Unfortunately, it is not illegal at this time. Your daughter may have a freedom of speech issue here where the employer is attempting to dictate whom she can converse with and be friendly with after work hours, but to date, there have been no cases that have come before US appeals courts to decide these issues. There have been a few minor cases where employees have actually said things about their employer or their employment situation on Facebook that have not gone anywhere for the employee because the judges have ruled that employers have a wide latitude to terminate employees because the employees are employees "at will" - meaning that they can be terminated for any reason or no reason. However, I have not heard of a case yet where an employee has sued for something like your daughter has experienced -- simply curbing the freedom of speech and freedom of association like that. She may want to see if she can get any local First Amendment lawyers interested in speaking with her and perhaps taking a case -- she can find one by contacting the local county bar association and describing the case and telling them what type of lawyer she is seeking.
I wish I had a more definitive answer for you but because these social media outlets are all so new, it will take many more years for these cases to work their way through the courts in the US.
I found more online myself than you are giving me for a fee. There was information on the National Labor Relations Act and much more. I expect a better, more detailed answer than "there may be a freedom of speech issue" or "I wish I had a more definitive answer for you"
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