Windows: Facebook is a time suck. Using it even once can potentially derail your productivity. If you need proof of how much time you waste on the site, TimeRabbit can show you.
TimeRabbit tracks your usage of Facebook -- as opposed to simply having a tab with Facebook open in it like many of us do -- and provides you with big blue numbers for the day, as well as storing information for previous days of the week.
It won't actually keep you off the social network, but hopefully seeing how bad the problem is will provide you with some motivation to close that tab.