It all depends on your office culture, really. There are couple of things you should ask yourself before you disable that access.
1. Do you believe that Facebook is negatively impacting your employee's performance ?
2. What will come to replace facebook (and, perhaps a dozen of other social networking sites) once you deactivate it ?
You see, if you really have an issue with employee performance then facebook is likely an issue with the work ethic at the workplace and thus disabling access to facebook really doesn't address that issue. What you want is for your employees to focus on work while they know that they have access to facebook.