Could you explain? I hardly ever use Facebook so I can’t remember the old one but the new one seems to be perfectly fine. It’s snappy (the page doesn’t reload and it seems to be faster than flickr), photos are displayed sufficiently large (still apparently no way to download originals, though) and the arrow keys work. All the things that make me happy when I encounter a photo viewer are there.
It’s nothing special per se but a perfectly fine implementation of a photo viewer as far as I can see.