Personally, social media is a chore to keep up with most all of them are walled gardens. You have to reply to Facebook messages on Facebook, you have to reply to MySpace messages on MySpace, etc. I currently have about 550 contacts in my email address book, less than 1/8 of those people do I talk to on a regular basis. If I were to add all my social networking contacts into those group I'd probably close in on 5,000 but I find it a chore to log in to all these social networking sites just to communicate with people. Not to mention that just by logging in I'm revealing my browsing habits and demographic information. Joe's thread about privacy is something to think about.
Notice how all social networking sites never reveal your 'friend's' email address? Heaven forbid you contact someone outside of their service, they might loose page views and thus advertising dollars. When people finally open their eyes and realize that EVERYONE of these social networking sites are only interested their demographic data and browsing habits so that they can squeeze more money out of advertisers, people we realize the system isn't so great.