In general, an employer will only be able to monitor peripheral data (such as the point in time of the communications or interactions, their length, and the involved connections). Monitoring the actual content of communications requires outstanding interests, which the employer will not be easily able to show. As regards telephone communications, they are in principle protected by criminal law.
An employer may control social media in the workplace if it is necessary for the performance of the employment contract and further is proportionate. Under these conditions, an employer may block social media completely.
In contrast, it is rather unlikely that an employer is able to show a legitimate interest in controlling an employee’s use of social media outside the workplace.