The fact is that most employment is what is known as "at will". This means that a company can set the conditions of work much as it sees fit. This includes requiring employees to putting a social media site on their personal devices (whether or not it can be unblocked). Further, failing to do so can result in suspension and/or termination. This is true unless there is a union agreement or employment contract that prohibits this action, or if your treatment would constitute some form of legally actionable discrimination.