Companies should not intruded into privacy of employees at all. The employees should from onset be made aware that selling of company's intellectual property and other secrets of differentiated competitive advantages is unethical and punishable under the national laws. Yes employees may use responsibly and professionally use social media in private communications. For the company's business promotion and advertisement social media remains indispensable medium in reaching out customers and establishing their preferences.
It is fairly acceptable practice as for some people social media is inevitable part of their life. Such individuals are clueless about what and what not to be shared on social media platforms. In fact, posting sensitive personal information in social media is commonly witnessed nowadays. But, beforehand such practice is followed by an organization, employees must be duly notified as not to do so and negative consequences if found guilty.