I think log-normal distributions usually fit this kind of data as well as power laws do. Anyway, such a plot is not the best way to choose between those two models (see http://arxiv.org/abs/0706.1062). We don't actually do any such test in the paper though (or say that it is a power law or log-normal). You might want to check out the paper for more details. https://www.facebook.com/publications/244240069095667/