The contributions of industrial economics to the competitive analysis of digital network and service markets.

Summary This thesis brings together the main milestones for the competitive analysis of markets involving network activities and digital services or products. It is organized in three parts, which are attempts to account for the impact of the specificities of these markets on their competitive dynamics. The first part of the thesis deals with the issue of the concentration of Internet platforms. A two-sided market model is specified to account for the different levels of interdependence on the platforms linking companies or advertisers, on the one hand, and audiences or consumers, on the other. The result is that network effects (synergies and economies of scale resulting from the number of platform users) are such that any advantage (first entry into the market, higher quality or relative audience share, etc.), ultimately constitutes a sustainable competitive advantage. The second part of the paper addresses the question of the delimitation of "relevant markets" in the context of mergers applied to the fixed Internet access market. The structure of demand for Internet access is examined econometrically in order to verify the existence or absence of segmentation (or even differentiation) depending on whether the technologies used are broadband or very broadband. The third part deals with the effect of ad-blockers on media and content providers whose business model is based on free access. A two-sided market model is specified in order to verify what role Internet service providers can play in internalizing network externalities and limiting congestion phenomena. Descriptors: digital economy, competition, platforms, concentration, relevant market, advertising, network externalities, media.