This update to the Facebook photo viewer comes a year after the last major overhaul in February last year when Facebook introduced a light-box like effect to its photo viewer and also made viewing and sharing easier.
Last year's upgrade was necessitated because the code used was obsolete and also for the need to support high resolution images. This time around it seems to have more to do with enhancing the user experience.
Google+ offers a similar feature: When opening photos in Google+, users will find a sidebar to the right of an image allowing them to comment or tag people. The layout isn't identical to Facebook's, but they are strikingly similar.
