I've always found this to be a muddled claim. Phil of Science isn't my thing but claiming an entire school of thought with its hands in just about all of the humanities as 'unfalsifiable' is just not really useful in 2015.

I'm tempted to ask, unfalsifiable in regards to what? As a theory of progression of history? Well, in that respect Popper is right to claim that Marx's specific predictions were incorrect. But the very nature of the theory of history Marx is proposing leaves plenty of open avenues. Marx's appreciation for technological development under capitalism is well known but it's not clear even he could have seen just what would occur in the century following the publishing of his work. Following that it's not clear how he'd reconfigure or 'update' those predictions following our current situation. At times it seems like you need to know that technological development has 'maxed out' to even begin discussing whether he was right or wrong.

That's in my opinion, conflating a handful too many issues to be all that useful. Marxists have been developing theories within philosophy, sociology, economics, literature, history and so on for so long that separating out the falsifiable from the non-falsifiable seems too complicated to make the over-arching theory dismissable on those grounds. And even then, assuming we could dismiss it that casually its function as a critique of existing theories, where it pushes them to expand in scope or challenges some assumptions at play would pull its relevance forward.

That all said, it's worth noting that (as far as I understood Popper's falstification criteria) Popper was trying to demarcate science from non-science. Not "bullshit from non-bullshit" in the way we think when hearing these terms. A theory might provide insight while still being falsified as a bonafide predictive science.

As I mentioned, I'm speaking out of my depth in terms of Phil of Science so take those comments with a grain of salt.