A bit of a rant:
Whenever the disappearing public sphere is discussed, inevitably consumerism is brought in, because it's the incursion of the market into matters once dealt with politically, through civic conversations, that's ruined civic life. And the recent reconceptualization of public space as shopping space first and foremost must be considered: is it that consumerism has become so prominent that it was forced to annex that public space (the utopian Main Street of nostalgic lore), or was the conversion of that space what has led to an expanding consumerism? Is it that people tend to express their opinions, to have a public voice primarily through their purchasing decisions, through their practices as a consumer? That they are most empowered when grousing for their "rights" as a consumer, as if they have any? (This idea is explored in Lizabeth Cohen's book about consumerism and politics in the 20th century -- more on that later). As we become convinced that buying matters more than speaking, we lose our ability to use language precisely, to spit out anything coherent that's longer than a slogan, to use logic that's not as elided and misleading as that of ad copy. And we forget the kinds of autonomy that don't revolve around acquisition.
The public sphere disappears because the economy so rigorously impresses on us the need to revel in our private purchasing choices, and see these as the essential building blocks of selfhood. Consumer-driven capitalism thrives on individuation, on every individual making purchasing decisions for herself. Economic expansion seems to correlate with the decentralization of decision-making -- so that now American families hardly buy anything as a group. Sharing a purchase implies sharing the decisions about purchasing of it, and sharing that means surrendering an opportunity to define oneself in the way we've been encouraged. Since we've been convinced we are what we own and not what we do, what we buy is basically who we are. If we let someone else buy for us, we become nothing, no one.
The public sphere, if it still existed, would theoretically be a place where dissenting voices could confront each other in an atomosphere of mutual respect. But as more of our experience is remade in the image of market-driven consumerism, the less we tolerate dissent. We expect our experiences made to order, without the spontaneity conflicting visions might provide. Internet technology -- the proliferation of social networking tools and online personals and such -- makes it possible to restrict one's communications to only those who share your rigid set of priorities and predilections; you can filter out everyone except those who already agree with you about everything, and thus you can have perfectly narcissistic conversations with yourself for the rest of your life. These identi-friends will match you in your hobbies and shopping preferences, reinforcing the idea that these are the sum of you. Rather than entertain conflicting notions, we withdraw into solipsism, calling it "diversity" when we refuse to try to integrate differing points of view regarding complex matters of shared social reality. Consumerism, and its attendant ideological crutches, obviates the possibility of a community. There can only be a market, in which we are all by definition competitors. Worse, we are thrown in to a marketplace of self-definition, deprived of the social/community arenas where we used to be able to define ourselves and our purpose.