Instead, since it's not every day that a twitter argument with an overcredentialed buffoon leads one to clarify one's thoughts about matters of importance, I would rather jot down those thoughts while I can.
The claim of Marxism to being a scientific description of the motive forces of human social action died somewhere around the time that atmospheric concentrations of carbon dioxide reached 350 parts per million, i.e., some time in the late 1980s. Its fundamental hypothesis that capitalism would pave the way for communism on the basis of material abundance was premised on another, less explicitly articulated hypothesis, as to the relationship between human interests, reason and action, a fundamentally Spinozist view that once people know what is to their rational benefit, sooner or later they will get around to acting accordingly. We have had a quarter century or so to see whether this would ultimately be borne out, whether the passing from the scene of the pseudo-Communism of the USSR would open space for the actual communism of an activated working class. In that time, capital has further eroded the preconditions not only for its supersession, but even for human survival.
One of the internal challenges to Marxism's self-conception, however, was its own theory of ideology, which described in fairly comprehensive fashion the various mechanisms by which class-divided societies make it difficult, if not impossible, for those who live within them to accurately discern their nature. The degeneration of certain types of Marxism into a toolbox of techniques for ideology-critique itself became a mechanism for the ideological defense of the capitalist system which their proponents had lost either the ability or the will to either comprehend or transform. The apparent antinomy between Marxism as a promise that a scientific understanding of capital could be attained through the struggle against it, and Marxism as an alibi for the apparent all-pervasiveness of capital's counter-scientific self-misunderstandings, was not irresolvable in principle, but it was not resolved in time.
Given that history, it would be easy to conclude that a scientific understanding of human society is impossible. "Philosophy persists because the moment for its realization was missed," to paraphrase Adorno from memory. Easy, intellectually lazy, and defeatist.
Rather, it is possible to understand human society ecologically, as part of a natural process, as the accumulated behaviors of an animal species whose capacity for ratiocination serves primarily as a means of communicating post-hoc justifications for an ensemble of instinctual responses. Doing so requires actually listening to, and learning from, natural scientists. It means overcoming the tendency--most clearly enunciated by Hegel in his Encyclopedia Logic, and brought most explicitly into Marxism by Lukács--to regard nature as a fundamentally unchanging set of cyclical processes and human activity, whether defined as spirit or as labor, as the true source of dynamism. Rather, it is nature that is the dynamical system, responding in non-linear fashion to small changes in forcing conditions, whereas human society continues to be dominated by its own accumulated momentum, the momentum of accumulation.
Crude empiricism? Perhaps. But it's all we've got.