I view Facebook and Twitter as unnecessary playgounds for me. My objective is to limit my channels of communication so i don't have to micromanage all these unproductive services. I don't see why you'd want to limit your user base like this.
Then sites started requiring emails to sign up - and you signed up anyway. Why wouldn't sites just let you register through snail mail? They're limiting their user base!
In all seriousness, Twitter and Facebook are trending, and you might as well adopt them. Not doing so would strike me as a bit of technological ignorance.