Given the limited resources of wireless sensor network infrastructure, knowledge of the traffic generated by each node and service can be of great value. Yet, accounting and conveying this monitoring information in this low-resource infrastructure is challenging. Current monitoring solutions for wireless sensor networks either use passive monitoring, requiring a separate network, or focus only on detecting and debugging failures in the sensor network. This paper presents FAMoS: a flexible active monitoring service to collect wireless sensor network traffic volume and distribution data. The service is provided with limited overhead and is applicable in many contexts. Each node locally collects data about network traffic and then periodically transmits the data to a back-end for further analysis and processing. A prototype for the Contiki operating system is presented and evaluated in terms of runtime, memory and communication overhead.