If a culture in your office develops around excessive instant messaging in the workplace, the solution isn't to cut the use of this productive tool or to institute monitoring of the service. That just makes employees irritable, and encourages them to spend resources avoiding supervision rather than on doing their jobs. If workers are wasting time, it's a sign that they aren't being challenged sufficiently - or work is being unevenly distributed across the work force. Encourage workers to use their status messages to keep everyone aware of what each person is up to in the office.
The impulse of users will be to fill these status messages with personal information and perhaps what music track they're listening to, but it's far more useful in an operation sense to keep professional chat addresses devoted to work purposes. If a culture develops in that workers are happy to share with one another the tasks that they're working on in real time, it encourages a sense of pride in their work which in turn enhances productivity.