Facebook is the grown-up keep-in-touch site.
Just kidding.
Seriously though, I used to have a myspace back when I was in high school, so maybe thats why it seems a bit immature to me to have one. No offense intended, just my opinion.
I have my cardomain page for my truck, my facebook page for my college friends, and Windows Live Messenger for chatting real-time with those friends so I don't use my cell-phone minutes. That plus snail mail is all I need to keep in touch with those I want to keep in touch with.