Too much spam on Twitter, and untill they remove all of it, untill it is removed it will never replace facbook.
I like the way everything is all linked together on facebook, plus there's more room for pics and such, I don't too much like the quizzes though, I hated tests in school.
I think Twitter and facebook has different target market. Twitter is more on 20's and up while facebook is 20's and down.
I hate Twitter, a lot. Facebook is easier to communicate with Twitter just says what you're doing constantly only. Boring >>
from what wikipedia says twitter has a goal of having 1 billion users by 2013. just google twitter wiki and have a 5 min read.
I generate traffic from both via article bookmarking buttons on my site. I find twitter generates more targetted traffic in a short amount of time but then tapers off. Depending on where you post in facebook, you can have a longer residual effect, ie posting on a group wall with less traffic keeps your message up longer. Twitter users have low attention spans and stick to recent dates only.
twitter is instant..facebook is quality...and its an good idea to use both of them...though facebook is a lil bit ahead!