Why? Anything you do at work is open to your place of employment. I work in network/information security, and while we don't decrypt encrypted connections, we do log employee Internet access and use the data for investigative purposes. Why would an employee have an expectation of privacy from their employer while they are using corporate assets?
Yes, many companies have DLP (data loss prevention) systems what sniff all outbound data watching for information leaks. If you're posting on Facebook at work, it is very likely that your employer can see exactly what you're sending. We just don't care unless it's sensitive data (get back to work).