One hundred years of court case decisions have galvanized the marketplace of ideas as a central pillar of first amendment jurisprudence. John Stuart Mill, who is widely seen as the father of the marketplace, argued that no person knows the objective truth, nor does any one idea embody it. We are fallible and often stubbornly convinced of our own rightness. This human instinct can lead to censorship, and to untested dogmatic beliefs.  As Virginia Tech Communications Professor W. Wat. Hopkins writes of its legal significance today, "The marketplace of ideas metaphor is the model most called upon by the U.S. Supreme Court in the resolution of free-expression cases. Justices have used the theory in the adjudication of virtually every area of First Amendment law, despite increasing attacks on the theory. For the most part, however, the Court does not recognize a single, universal marketplace of ideas, but numerous mini-marketplaces, each with its own dynamics, parameters, regulatory scheme, and audience." 
The marketplace metaphor is still founded on a basis built in a bygone era. It does not translate to the modern, digital age.
[P1] The marketplace of ideas is a concept that has been consistently re-asserted by US judicial decisions. [P2] This would not be the case if it didn't work.