If you want something more than the RIM provided Facebook app for BlackBerry you may want to check out Facebook Messenger. It was just released today by Facebook and sort of replicates some of the features that RIM already added in Facebook for BlackBerry v2.0. It is a simple interface for using Facebook chat and messenger directly from one app. I am not sure how redundant they are but Facebook has similar apps on iOS and Android.
You can pick up the new Facebook Messenger free at fb.me/msgr