May 28, 2018:
"Social Formation" is the mature Marx's concrete concept for thinking the complexly structured articulation within which Modes of Production are found in reality. You can think of it informally as a specific society in real history: the contemporary French "social formation" is today's nation of France. But note that Marx rejected "society" as a non-concept, an ideological not a scientific term. Formally a Social Formation is a hierarchically-structured complex in which at least two but frequently more Modes of Production coexist, in the relationship we're familiar with in which one Mode of Production dominates the others. Sometimes these are historical survivals such as landed property or small-scale artisanal production within social formations dominated by capitalism; other times they may be new modes of production emerging, such as the capitalist mode of production developing within feudal social formations. The basis of Marxist historiography is the "anatomy", I think Marx used that term, of social formations and their evolutions. The basis for Marxist political strategy is the concrete analysis of the current conjuncture of the social formation.