I have been using the app from twitter and facebook on my blackberry for a while. It's very interesting to see their difference, and in fact, they can learn from each other to improve the service.
What Facebook Can Learn from Twitter:
(1) shorter summary on the highlights of friends status, so that users can see more updates from friends just by scanning
(2) Remove the add friends button since people will rarely use phone to add friends.
(3) Also reduce the refreshing time. Twitter is much better on this.
What Twitter can learn from Facebook:
Facebook Mobile integrates into blackberry much better by having "facebook" on the navigation panel, and then it stopped sending me msg via email. While for twitter, it still sends me msg via email though I can get it from the twitter app.