The code itself is poorly designed, the API's are all over the place, there are inconsistencies everywhere and the performance is clearly lacking. On the flip side, take a loop at the LinkedIn app for iPhone. It's super slick, easy to use but still has a ton of features and the performance is far better. Yes, it's not as slick as Path but its miles ahead of the Facebook app.
So if I were Facebook, I'd rely a bit less on the 100% platform agnostic approach, take it back a bit and build the things that make sense cross-platform with HTML and build the parts separately that make sense to build using native code. This approach to me accomplishes much of the same cross platform success without creating such a crappy and laggy app.