[O]ne might say that Marxism was once a science, but one which was refuted by some of the facts which happened to clash with its predictions[...]
However, Marxism is no longer a science; for it broke the methodological rule that we must accept falsification, and it immunized itself against the most blatant refutations of its predictions. Ever since then, it can be described only as nonscience—as a metaphysical dream, if you like, married to a cruel reality.
F.A. Hayek, who was greatly influenced by Popper and actually parted company from the extreme a prioristic approach of his mentor, Mises, made a very similar point here (see from 04:30):
Both the Marxists and Freudians had the dreadful habit of insisting that their theories were irrefutable; they [were] logically and absolutely cogent. And that led me to see that a theory that cannot be refuted is not scientific.UPDATE: Jon Catalan has some thoughts here. I left a comment underneath too.