Online graph database service providers have started migrating their operations to public clouds due to the increasing demand for low-cost, ubiquitous graph data storage and analysis. However, there is little support available for benchmarking graph database systems in cloud environments. We describe XGDBench which is a graph database benchmarking platform for cloud computing systems. XGDBench has been designed with the aim of creating an extensible platform for graph database benchmarking which makes it suitable for benchmarking future HPC systems. We extend the Yahoo! Cloud Serving Benchmark (YCSB) to the area of graph database benchmarking by creation of XGDBench. The benchmarking platform is written in X10 which is a PGAS language intended for programming future HPC systems. We describe the architecture of the XGDBench and explain how it differs from the current state-of-the-art. We conduct performance evaluation of five famous graph data stores AllegroGraph, Fuseki, Neo4j, OrientDB, and Titan using XGDBench on Tsubame 2.0 HPC cloud environment.