Can we not be trusted? Have you ever thought that when your corporation blocks access to social networking tools such as msn or facebook they are restricting your ability to problem solve quickly.
IT professionals are usually hired not only for their skills but additionally for their network of people. In restricting this access, then your inhibiting their ability to do what they've been hired by you to do!
Can discussions outside of work really be limited? how about emails from home? they are still subject to the discretion of the employee according to the contract he would've signed on joining the company. So why do we believe that by social network tools at work that we would be controlling what is said?
Then again if its about inhibiting productivity then are you sure you've hired the right people? Can they not be trusted to deliver on their word?