Facebook is currently testing a new photo lightbox. Photos has been one of its most popular product to date so any change to it is carefully weighed. Those that have spotted the new photo viewer in the wild noticed something interesting, it is very similar to the one on Google+.
The layout is the same, comments on the right rather than below, a dark tint around the image, rather than the white one Facebook has now.
The Google+ lightbox is very similar to this, albeit it goes for the full black background.
Google+ was "inspired" by Facebook quite a lot, but the giant social network returned the favor with a few changes and features that were inspired by Google+.
That's actually a good thing, competition leads to better products. The new Facebook lightbox is the perfect example, while similar to the Google+ one, it looks significantly better.