Facebook has been updated to version 3.1.4, which is now compatible with iOS 4. Facebook has not added many features in this update, and seems to be released solely for the compatibility with iOS 4 and iPhone 4.
Facebook 3.1.4 for iPhone/iPod Touch allows speedy switching between apps, and high resolution icons for iOS 4. Now, it also supports Korean and Russian localization. There are some minor bug fixes as well.
Facebook 3.1.4 is available on iTunes AppStore for free.