Thomas Hawk does a cool Google+ vs. Flickr vs. Facebook vs. 500px vs. Twitter comparison.
I will try to compare, the best I can, the attention that the same photograph received from each of these sites over the course of 24 hours. If as a photographer you are looking at photo sharing sites, in part, as a way to promote your work to a wider audience, the engagement your photographs receive online may be of interest. This case is very specific and of course everyone's circumstances will differ, but this is my experience.