What's the easiest way to limit time spent on social networking websites?

I’ve been a stickler for social networking websites, and lately it’s beginning to control a lot of what I do. I read in an article that the best way to stay happy and active is to get off the computer. That’s sort of the issue I’m having at the moment.
Are there any strategies that will help limit the time I spend on these websites? (Ok, I know being on this website isn’t a good example).