I completely understand what you are saying.
However, there are several things to consider.
First, think about who you are connected to on each social network. Some people want to share things only with specific people. Therefore, you might have a social network that you use to connect to coworkers (LinkedIn), one for family and old high school classmates (Facebook), one for real-time updates and sharing ideas and articles that you find with anyone (Twitter), one for your friends who share a specific hobby (Pinterest), and one to share the places that you have been with only the people you trust with that information (Foursquare). And, younger users might want to have a social network where they can share their party photos with friends that they don't necessarily want their coworkers or family to see. (Each person might use the social networking sites that I listed differently. However, I know that I am connected to different people on each social networking site and it influences what I share on each.)
Second, we are living in a more mobile society. Therefore, it is often very difficult to see your friends and family, in person. I think social networking sites actually help make the real world connections stronger. (In theory, at least.)
Third, social networking sites make it easier than ever to meet new people all over the world who share similar interests. (HubPages and Twitter are great for this.)
As with anything, moderation is key. All good things can be bad if used too much.