For the most part social networks give you the tools to manage friends fairly well. I know on Facebook you can change a setting to only view important updates from people. I like this option because you don't have the social awkwardness of unfriending someone. But, you don't have to constantly see what they at for lunch.
Also, I think we just get bored with things. That's human nature. We like new things. We want to be part of cool new trends and technology. So once something hits mainstream and stops being cool, we start looking elsewhere for the next cool service to try.
Having to manage an externalized representation of your relationships is what is against human nature. Feeling social pressure to check daily, weekly, or even monthly on the activities of every person you may have ever met is what is against human nature.
Also, I think we just get bored with things. That's human nature. We like new things. We want to be part of cool new trends and technology. So once something hits mainstream and stops being cool, we start looking elsewhere for the next cool service to try.