Chrome: The web is full of distractions, but none are more distracting than Facebook. If you find yourself particularly drawn to wasting time on Facebook, this Chrome extension will block access unless you have notifications.
If Blocking Facebook entirely seems like overkill to you, Facebook Nanny is a good middle ground. If you have notifications, it'll let you respond to them, and you can always visit your profile or send messages if you need to. But if you don't have anything waiting for you, it will block you so you can't just sit around on your News Feed, where the real time wasting takes place.
Facebook Nanny is a free download and works wherever Chrome does.