Friend sends me a bit of email saying, sign up for LinkedIn/Facebook/Orkut/Myspace. Doesn't matter which or who, I've got requests like this for all of those from various friends, some more than once.
I sign up, laboriously.
Same friend sends me a LinkedIn/Facebook/Orkut/Myspace message/scrap/add-request/writes on my wall, saying "Here's an interesting way to stay in touch."
Why? Friend had my email address, right? And I have friend's email address, right? So why would I want to give up the considerable hassle of sending email back and forth, for the pure simplicity and convenience of connecting to a website, signing in (using my email address, more's the irony), clicking on "scraps" or "wall" or whatever, and then reading a message that says "Heyyyy-y dude! how're things?"
Yeah, I haven't figured it out either.