When I want to keep in touch with someone college-aged, I don’t ask for their phone number. I ask them to facebook me.
That’s shorthand for “add me as a friend to your facebook profile.” Sometimes, I give them my name and they would have to search in facebook to find me. More often, I give them my name and tell them to search a common friend’s friend list.
The friends list is a usually public list of everyone you share friend status with. So when I meet Doug through my friend Bob, I don’t have to remember Doug’s full name. I just remember his name, or even just his face (most facebook users put photos in their profiles), and look through Bob’s friend list to find him.
If facebook went down, I would not know how to get ahold of many of my friends. I some of their cell numbers, but for some people, leaving a message on facebook is at least as reliable as calling them.
If you are involved in marketing to people under 30, read up on facebook and myspace. Understanding how your target audience communicates is crucial for crafting spreadable marketing messages.