Twitter is useful for me as an aggregator for gathering twitter posts by people I'm interested in. All of my family members who do use twitter also have the FB<-->Twitter thing hooked up so I'm not missing anything from them by ignoring twitter.

Personally, I find very little value in twitter. Maybe it's because I'm in the wrong demographic or my social circle doesn't use it, I don't know. I could imagine a world where we posted everything through twitter, but in reality almost nobody I know personally actually uses twitter.