PayPal prvoides an online transfer money network. Each payment flow connects senders and receivers into a giant network where each sender/receiver is a node and each transaction is an edge. Traditionally, the risk score of a transaction is computed based on the characteristics of the involved sender/receiver/transaction. In this talk, we will describe a novel network inference approach to calculate transaction risk score that also includes the risk profile of neighboring senders and receivers using Apache Giraph. The approach reveals additional risk insights not possible with the traditional method. We leverage Hadoop to support a graph computation involving hundreds of millions of nodes and edges.