Don't friend subordinates. This may seem to fly in the face of the previous bit of advice, but it must be said nonetheless. Some companies encourage supervisors to friend their subordinates as a means of building unity, breaking down barriers and making managers seem more approachable. Bear in mind, however, that you're also invading a personal, relaxed space and polluting it with business standards of decorum and stuffiness. Chances are also good that your employees will feel spied upon.
Consider, too, the exposure you risk by potentially mixing your subordinates with your high school friends or drinking buddies. You might be one racist joke or embarrassing photo -- posted by someone you friended out of reflex -- from losing the respect of your superiors and of the people who report to you, too. When that happens, human resources and the company lawyers might not be far behind.