I usually friend people because of mutual interests. I unfriend them when we no longer have mutual interests and I don't see anything from them in a while. If someone posts the same thing on Facebook, Twitter, Google+ and LinkedIn, I'll just follow them on one channel.
If they are obsessed over their cat/baby/motorcycle, I'll just hide them for a while. If they become interesting again, I'll unhide.
The worst offenders: teenage nieces and overly political in-laws 🙂