Facebook is such a mess. It's popular but a total mess to work with. Too many features, a beyond-cluttered API, and one of the buggiest, most confusing UI's I've ever seen.
Contrast that with twitter's clean, simple interface, easy-as-pie API, and open acceptace of 3rd party applications (twitterific, tweetdeck, twitterfon, etc).
No matter what FB tries to steal from Twitter, it'll still just be Facebook, a cluttered behemoth, packed with useless features and mildly annoying advertisements.
(That's my opinion on the matter. Now I gotta go update my facebook status…)