Regardless of your industry or business, there is a very good chance that your employees have more to do on their desk and their computer than they can get done today. I am old enough to remember work without personal computers, cell phones, or text messages.
Our communication tools create new tasks and interruptions at lightening speed everyday. So, how does your team prioritize what they are going to get done today? In the absence of clear priorities there will be a tendency to just stay busy without regard to the business implications.
First, make sure your employees know your priorities and how they support those. Otherwise you can't be upset about what they choose to do on their own. Second, encourage your team to huddle at least once in the morning to make sure everyone knows what the priorities for the day are. Don't let them hide in their cubicle.
Third, every few days, huddle with your team and review what has been done and see if any critical priorities were missed. It takes effort to keep your team focused but failure to do that costs much more.