Facebook is continually work on the new design and provides best features to their users. So to moving on the direction they are testing the new design for photo viewer.
This new photo viewer displays comments and Likes button on the right hand side of the photo rather than below images i.e in simple you can say that this new photo viewer is very similar to the Google + Photo viewer.
This is not first time when Facebook adopted the Google + features, In Previous Facebook also added these Google + Features asymmetrical subscribe option, video chat, friend list, post length and now photo viewer.
The new Facebook photo viewer also added advertisements to the Photo Viewer, which is annoying for most of the users to see ads alongside with their photos.