That's not true, there are plenty of things that can be measured in social science: the frequency of certain cultural behaviors, the correlations between population demographics, etc. As with any science, there are theories which are well supported, and hypotheses which are not. You have to address the specifics to say something meaningful about whether or not it's "conjecture".

Science is all about improving observation and measurement in order to be able to make reasonable conjectures about cause and effect, action and reaction, predictability and probability. To the extent that social sciences can and do "improve" in those qualities of observation, measurement and prediction, I think it's safe to call them sciences. We seem to think we know a lot about chemistry and physics, to name two of the "hard" sciences. But we thought a hundred years ago that we knew nearly all there was to know about them then. And we know vastly more now. To take up the points you argued with HasntBeen, medicine is by no means a cut and dried science. There's so much we don't know about hard to measure effects such as the placebo effect, hypnosis, psychotherapy, etc. Not to mention nearly all of the category of "mental illness". We're in the Dark Ages still -- and yet we think that doctors are very "scientific". Well, compared to lay people, they are, even though they know so little. And archeologists can date bone fragments with a fairly high degree of accuracy, whether you want to believe that or not. Just because you don't seem to like (or understand?) their methods doesn't make them "wrong". And their tools are getting better, too. As for economics, Milton Friedman is so much more "right" than Karl Marx, that it's like comparing a brain surgeon to a witch doctor. Marx was a total fraud.

Even hard science is conjecture... it's merely extremely well-confimed conjecture. The very model of scientific enquiry uses an invalid syllogism, but it's the best tool we've got. There is no such thing as a "proven" theory. A theory can be condidered CONFIRMED, but can always be refined or dismissed as new information comes to light.