Need to look busier? Make sure your office appears swamped at all times. Your desk should always look like you're in the midst of some incredibly time-sensitive project. This way you can safely maximize the time you spend checking your facebook. 
For added protection, make sure the papers on your desk are arranged in stacks. A calculator, a few stray apples and a strategically placed pair of eyeglasses will round out the organized chaos effect.
Finally, some work on your computer screen that you can immediately access away from facebook when your boss checks on you. A carefully placed sandwich remnants will do much to make you appear to eat lunch at your desk, making you seem too busy. Be sure though, to throw it in the trash when you're off, otherwise insects may catch on you.