The current version of Facebook allows you to download your friends' photos to your device by default, so Facebook Photo Downloader has really become obsolete. Despite this, the app still works, and you can use it to save your friends' photos in a folder on your device.
Facebook Photo Downloader is a tool that has fallen into disuse, because there's no reason to use it with the current version of Facebook. It's just one more example of how quickly apps and technology in general can fall behind the times these days.
Requires Android 2.2 or later.