Let's not forget these are just tools. Most of the relationship people have on Facebook are as shallow as they get because it's easy to shoot a message on such systems. But do you really care about EVERYONE you have in your networks ? Would you really miss something crucial in your life if you didn't have it ?
Before people used to bond because they had things in common (Usenet, IRC with specific topics of interest), the "new" social networks are just about gathering numbers and bonding with people you have just met once or twice in your life and with you have little in common. At least, that's how I see most people around me using Facebook.