With the increased prevalence of advanced mobile devices (the so-called \smart" phones), interest has grown in mobile social ecosystems, where users not only access traditional Web-based social networks using their mobile devices, but are also able to use the context information provided by these devices to further enrich their interactions. In complex mobile social ecosystems of the future the heterogeneity of software platforms on constituent nodes, combined with their intrinsic distributed nature and heterogeneity of representation of data and context raises the need for middleware support for the development of mobile social applications. In this paper, we propose Yarta, a novel middleware designed for mobile social ecosystems (MSE), which takes into account the heterogeneity of both deployment nodes and available data, the intrinsic decentralized nature of mobile social applications, as well as users' privacy concerns. To validate our approach, we show how we developed two mobile social applications over Yarta, and report on both its e ciency and ease-of use by way of extensive evaluation on smart phones and laptops.