The first thing to understand is that Facebook is all about “the social graph” while Twitter is about “the interest graph.” For the most part, you “friend” people on Facebook because you’ve met them at some point in real life. Twitter is about reading and engaging with interesting people, whether you know them or not. The whole point of Twitter is find people with interesting stuff to say, and tune into their train of thought. If you’re waiting for your friends to join Twitter, you’re misunderstanding what the service is for.
from A Guide to Twitter for Facebook Users — scot hacker’s foobar blog the best intro to twitter I’ve seen.