With the occasion of launching the iPad app and the redesign of the Facebook mobile webpage, the iPhone client has been updated to a completely new version, number 4, often referred to in the past as Project Spartan.
Right from the beginning, if you are having problems starting the application after the update (like we did), just uninstall Facebook and reinstall the new version.
That small glitch aside, the app looks really good! We wished sliding gestures would be enabled (like on the iPad) to reveal the side menu instead of pushing the small button. So what does the new Facebook bring? Improved search and browsing, Games and Apps as well as better Chat, Navigation, Bookmarks, Search and Security. Follow the source link to grab it or check with your iTunes or App Store apps.