There is actually a mroe recent trend that's begun to occur recently that utilizes the main purpose of social networking sites, the networking, while downplaying the personal relationship building. Rather than unfriending someone, simply change how much they pop up in your feed. Go to their page and change the settings from "most posts" to "no posts" and you won't see anything from them again.
The reason this trend is becoming popular is for future networking. Say the person you want to unfriend goes on to get a job at a company you would like to work for. If you didn't unfriend them you have an easy connection to get an inside scoop on the jobs offered. Just send the person a quick "how have you been" message and go from there.
Personally, sites like Facebook and Twitter do more to harm personal relationships than they do to build them. Rather than approaching the sites as a primary means of maintaining a relationship with people, use them strictly fr peripheral and passive information and creating a large network that you can work with down the road.