Since the 2000s, numerous start-ups and agencies have argued for the necessity of analyzing social media data to "know what people think", as they are deemed to provide access to spontaneous expression of thoughts, tastes and representations. How do these actors, and the various types of knowledge and technology they draw upon, change the way we know and act upon people's opinions? This paper offers insight on these understudied actors, by describing the emergence in France of a market for measuring online opinion. It shows two distinct trajectories of innovation, and the key role played by the early clients of these companies and by the demand for tools for online reputation management in the shaping of these instruments, and the definition of epistemic value. Both approaches of online opinion break with the classical egalitarian conception of public opinion. They instead conceive opinion as a mediated and collective process in which not all opinions have an equal value.