Their mobile iOS application is absolutely the worst application I have ever used on iOS.
It just doesn't work far too often (it took 2 months for notifications to randomly start working on my new iPhone 4S, but activating them can't load the new content half the time or just sits there loading forever; I have no network issues with any other iOS applications) and irritates me too much (e.g. I keep getting stories from pages I hid on the desktop site, you can't manage photos properly like on the desktop site, etc.).
I've reverted to using the desktop site under mobile Safari but I'm dying for somebody to replace Facebook simply with something of higher quality - I don't mind the ads or tracking and I genuinely find it a useful site, it's just of such poor quality it's annoying!