AppStore application.
I'm sure Facebook will come out with an iPad version (they better). All in all, the AppStore applications are more robust and optimized than the full site. Even if the features are the same, the very fact that most of the UI elements are done by the device while only information is through the network theoretically makes the AppStore applications faster.
Twitter for me would also be the same. For Twitter, though, even the iPhone/iPod touch version is good enough for me as the most I ever do with it is post a tweet.