The new Facebook app is superb. There is really easy toggling between Feed, Profile, Friends, Inbox, etc., and the thing works perfectly for browsing photos. It's beautifully laid out, and great for uploading photos as well.

Facebook tends to do things the right way.

Facebook has built a very nice application, and the photo slideshow has always been awesome...however they failed to add the ability to add photos from your "Photo Library".  Currently you have to take a new pic in order to post one.  :-(

Really simple and effective iPhone application that manages to work even better than their iPhone web site.  You can set your status and post photos right from your iPhone camera.  Also includes Facebook chat, which makes it an Instant Messaging program, too.