The people who don't consider sociology as a science, do think the same about anthropology. They mostly see as science just physics, biology and chemistry, and related fields. If based on methodology, sociology uses mathematical tools and it could be considered as more close to science than anthropology which basically only uses qualitative methods. Popper's "we cannot know" is a philosophical view, we cannot know anything for real. We cannot be sure about gravity, about energy, light, colors, anything. Even the sciences based on empirical data are not sufficient to claim that they are universal truths. So, basically, Popper smashes modernist view that there is a truth in science. Sociology is falsifiable. Everyday a lot of articles are written about false predictions, false research methods in psychology, sociology. It is just these are more complex than physics, because of the subject that is humans.