We study a game-theoretic model for the diffusion of competing products in social networks. Particularly, we consider a simultaneous non-cooperative game between competing firms that try to target customers in a social network. This triggers a competitive diffusion process, and the goal of each firm is to maximize the eventual number of adoptions of its own product. We study issues of existence, computation and performance (social inefficiency) of pure strategy Nash equilibria in these games. We mainly focus on 2-player games, and we model the diffusion process using the known linear threshold model. Nonetheless, many of our results continue to hold under a more general framework for this process.In more detail, we first exhibit that these games do not always possess pure strategy Nash equilibria, and we prove that deciding if an equilibrium exists is co-NP-hard. We then move on to investigate conditions for the existence of equilibria. We first illustrate why we cannot hope that games over networks with special in and out-degree distributions -e.g. power law -are more stable than others, concerning for example, the form of the improvement paths, or cycles that they induce. We then study necessary and sufficient conditions for the existence of pure Nash equilibria, both for the general case but for some special cases as well. Our conditions go through the existence of generalized ordinal potential functions. We also study the existence of -generalized ordinal potentials (which yield -approximate Nash equilibria) and provide tight upper bounds on the existence of such approximations. Finally, we study the Price of Anarchy and Stability for games with an arbitrary number of players. We conclude with a discussion of the effects on the payoff of a single player (or a coalition of players) as the number of players increases.