I don't live on Facebook. On average, I'll check it a couple times a day, and almost always on my iPhone. This means that when I do check it, I often have many messages to look back on. (I usually look back as far as I'd previously seen.)
Unfortunately, their display algorithm sucks. Any time a post, comment, or like comes in (or it just feels like refreshing), it completely refreshes the view and scrolls to the top of the list. Even if you are multiple load-sets of messages in, it will reload you back to the top.
It wouldn't be so bad if it would return you to your last position, or if it didn't take so damn long, even over wifi. But it is bad.
Facebook for iPhone devs, look at the official Twitter app. They handle it perfectly. I was amazed.
New messages are loaded above your current view without disturbing your view position.
Fixing that alone will make me happy. Aside from the stability fixes (which were very welcome and necessary, I could do without another layout change (which seems to happen on nearly every update.)