Nowadays networked mobile devices have become an imperative commodity in people's daily life. A recent report from Bell Labs predicts that, by 2020, the consumption demand for digital content and services on mobile and portable devices will see a global average increase of 30 to 45 times from 2014 levels---with some markets experiencing as much as a 98-fold jump. To meet this huge demand and serve users from all over the world, a number of global (or planet-scale) mobile technologies have been launched and received great attention, such as mobile cloud networking, wearable computing, high-throughput wireless data delivery, and location-based services. In the meanwhile, having more than one billion users worldwide, Online Social Networks (OSNs) provide a popular venue of innovation with many interesting and practical challenges. In particular, more and more OSN systems have expanded to mobile platforms, not to mention those large-scale mobile-centric social network systems such as WhatsApp, WeChat, Momo, Snapchat, Skout, and Swarm. For both academia and industry, utilizing the emerging mobile technologies to help people's online social interactions has become a vital trend.

The 9th International Workshop on Hot Topics in Planet-scale Mobile Computing and Online Social NeTworking (HotPOST 2017) is organized with the goal to bring researchers working on the intersection of mobile computing and OSN together to present and discuss their latest research results and ideas, thereby promoting the development and evolution of this area. All submissions on mobile platforms and OSNs related to architecture, design, simulation, implementation, modeling, analysis, algorithm, and measurement are welcomed.