After all, the intention is that information about you is to be made available to other subscribers. However, it does seem that there is reason to believe that a crime may have been committed or might be committed, the obtaining of information that could reasonably be linked to such a crime ought to be legitimate. Anything that I post to a service such as MYM should be kept private except for those with a 'need to know', such as potential dates, or law enforcement agents with probable cause, or system administrator with a responsiblity to maintain the reliability of the system. Exactly where the line is is secondarily important to setting a clear one. Because then I the employee have the option to dare to reveal something personal.
People do send personal email.
The employee attitude toward a workplace privacy policy, ot lack there of, would seem to depend the the size of the workplace and the probability of ever actually being caught. At the firm where I worked last summer, all email sent by several hundred people was collected in a single office. It may not have been deeply erotic, but it was still personal communication. This doesn't answer whether it is right for a company to snnop, but as a practical matter, it didn't really affect anyone. Okay, let's go back to the hypothetical.