While I, like many others would have hoped for Facebook to fix their app, I get why Facebook is doing this.
Blackberry users rave about BBM but iPhone users can never get in on the fun. RIM would never share BBM with any other platform because it's one of their biggest selling points. It's why many people still get Blackberry's!
So now Facebook comes in and gives all smartphone users a standalone messaging app with all their built in contacts, already signed up! RIP BBM
This also seems to be part of new app strategy for Facebook. A while back, a Facebook photo/camera app was leaked. It was meant to compete with the likes of Instagram and other camera apps. Facebook after all is the biggest photo network in the world, bigger now than Flickr.
It's totally possible that Facebook is planning to break up its functions into one touch accessible purpose built apps, visible right on your iPhone's home screen.
The next step would be to release a Twitter-like app for status' and newsfeed, where you can also access Profiles and walls. These 3 apps together would make up all the functions of Facebook without trying to do too much in one app.