Dear Sir or Madam:
The answer is probably yes. If you are using the company's equipment and it during your business hours (or even after business hours if you are working late), the company has the right to access your messages. The usual reason for this is that the company expects its employees to use the company time for company business and not for "amusement" as you noted. Further, it is a way to monitor compliance with any written email/social network policies that it may have. If you are concerned, you should contact your HR department and ask that they give you chapter and verse of such policies from the employee handbook. If in fact there is no such policy, them you may possibly have a first amendment claim. Even if there is no written policy however, it would be very difficult and expensive to pursue the matter.
The caution that I always give clients is that: (i) email/im are forever; and, (ii) don't use company equipment for personal matters.
Best of Luck
Michael J. Katz
Answered on Nov 18th, 2011 at 3:23 PM