As for the Facebook/Myspace/Twitter/Whatever generation, the part I don't get is not that they live out much of their social lives online. That's easily graspable. The part that boggles me is that, at least for many of them, they literally seem to want to be in touch with their social network every single minute. What does that mean for the future of Western civilization?
I'm also an old fogey at the age of 27; I was just a touch too old/too involved in my blogging to really get into Facebook/Twitter/MySpace although I am reasonably well connected on Linked-in.
But I think there is an easier explanation here --- younger people (teens to mid-20 somethings) have almost always attempted to be highly integrated into their social networks at any time. I remember my little sister being able to easily spend four to six hours a night on the phone with her friends when she was 13; I remember randomly going over to my boys' houses to kick back and just chill out when I was 18. I remember the great moments of simultaneous evacuation and clustering in dorm rooms at 2:30 in the morning.
As my life has changed, working full time, falling in love, getting married, learning how to mortar and grout the bathroom floor --- my social network has gotten smaller in the number of people that I actively seek to communicate with on a daily/weekly basis, but it is held together by significantly stronger ties. I spend much less time reinforcing the weakest ties in my network of friends now than I did five years ago, or even more notably, ten years ago. And while it may differ by degree of strength, I think this is a fairly common progression over time for most individuals.
So the interesting question will be if MySpace/Facebook etc. users overwhelmingly maintain their massive weak tie networks when they are signifcantly older. The less interesting thing is the continuation of aging into and aging off of these loose tie networks.