Most employees are hired on their ability or past merits, rarely are people hired on their work ethic and moral. While some employees may be perfectly capable of doing the job, they simply choose not to. Many times this is not a conscious choice but is simply due to their laziness.
The first step is to find out why the employee is not performing to their peak. Some common causes of laziness are:
Ok, so maybe Facebook is what you do when you are bored, and not a cause of the boredom, but nevertheless, finding the cause of laziness is paramount. There may be a personal problem affecting them currently. While you shouldn't lower your standards, you CAN be understanding (if the situation is temporary).
Having an informative one-on-one with the employee will help you determine the specific cause of why they are not performing as expected. If you approach the employee with a calm attitude with a general concern for their, and the company's, well-being, then they will respond favorably.
The cause of either boredom of lack of motivation is generally caused by a lack of personal goals or professional challenges. From here you need to explain that the lack of production has been noticed, and that you are here to help correct it.
Once the employee realizes (or is pointed out) their lack of production, they will often be helpful in formulating a plan to ensure that they become more motivated. Speak with them frankly about increasing targets and goals for them to reach, something that can be monitored to not only ensure motivation on their side, but to ensure that the work is completed for the company. Remember, that if you honestly care about their well-being, then they will respond more positively to them. So listen to them with an open-mind and an open-heart, remembering that you as well have surely suffered from a lack of motivation at one point or another. This is not a bad employee; it may just be a poor time for them. This poor time should be looked at as something you can help them with, and correct.
You may need to get creative in your solution to the boredom problem. Consider an alternative plan of action for the employee such as adding responsibilities if they feel unimportant, or lessening the responsibilities if they feel overworked and burnt-out. If possible, consider reassigning them to a department or assignment that is something fresh, and may bust them out of their boredom-slump.
Remember, to treat them how you would want to be treated. Do not jump to conclusions, sit them down and have an honest conversation. If the situation is not a temporary personal problem, then work-out a strategy with the employee to ensure that they are pulled out of their funk, and back in to their important role within your organization.