A large number of applications in the marketplace has been considered a critical factor in determining which mobile ecosystem holds the competitive advantage and eventually gains a dominant position in the market. This paper investigates the influence of developer multi-homing (i.e., participating in more than one ecosystem) in three leading mobile application ecosystems. Our results show that when regarded as a whole, mobile application ecosystems are single-homing markets. The results further show that three percent of all developers generate more than 80 percent of installed applications and that multi-homing is common among these 'keystone' developers. Finally, we demonstrate that the most installed content comprises small number value propositions. The results thus imply that attracting and maintaining keystone developers is more critical for the survival of a mobile application ecosystem than the overall number of developers and applications. Hence, the mobile application ecosystems market is unlikely to become a monopoly. The study advances the theoretical understanding of the influence of multi-homing on platform competition in two-sided markets.