I think that the opening sentence of the work speaks to the social transformation that Marx and Engels believes is critical to the embrace of socialism over capitalism. In the line, “A spectre is haunting Europe—the spectre of Communism,” Marx and Engels speak to the idea that the growing number of the proletariat is making the condition of capitalism a force whose days are limited. Marx and Engels believes that the social transformation of power being abused in such a direct economic and political manner will contribute to great dissatisfaction amongst the proletariat. For Marx and Engels, this social reality has always been there in terms of the relationship between those who own the means of production and those who are ruled by it, but capitalism has exacerbated this condition to a point where rebellion is almost inevitable. The social transformation of this landscape is one that Marx and Engels sees as moving from merely difficult to being seen as obscenely wrong. For Marx and Engels, this intense level of dissatisfaction is where "the spectre" emerges, something that signals a fundamental change of power and change of reality between what is and what can or should be.
The social-historical transformation of the 19th century that Marx and Engels were most adamant about was that perilously robust ongoing transformation of the bourgeoisie. They give a detailed and emphatic description of this transformation. They say the bourgeoisie began the decline of the feudal system--though there is some room for historical clarification on this simplistic statement:
The bourgeoisie, wherever it has got the upper hand, has put an end to all feudal, patriarchal, idyllic relations. (Engels and Marx. The Communist Manifesto)
They say, with more accuracy, that in the 19th century, the bourgeoisie has torn asunder all established relationships and is in the process of continually remaking them, giving continual "disturbances" to the proletariat along with "everlasting uncertainty and agitation." They say that all social forms are broken down and broken down again by the bourgeoisie as they (1) relegate the proletariat to greater and greater limitations as wage earners and as they (2) drive for greater and greater distribution of product and acquisition of materials and labor for their products. This is the adamantly central 19th century transformations Marx and Engels write about.
The bourgeoisie has stripped of its halo every occupation hitherto honoured ... The need of a constantly expanding market for its products chases the bourgeoisie over the whole surface of the globe. It must nestle everywhere, settle everywhere, establish connexions everywhere. (Engels and Marx. The Communist Manifesto)