While you can magically find photos with any given number of people on Facebook, clicking on them (in Google’s image search) lands on the page, not the photo itself. You have to scroll down to find it. As I have found out, to find “just” photos (or rather, pages with one photo only), you need to X-Ray in a more specific manner, namely,
site:facebook.com/*/photos/a (add your keywords).
Here is an example of precise people counting by Facebook. Another example (suppose I am sourcing in Healthcare):