This document proposes ComSoc, a relational topic model that adaptively transfers user behavior data across composite social networks to improve sparse user behavior prediction. ComSoc selects relevant social networks for each user and generates topics and behaviors. Experiments on real-world datasets from Tencent and Douban show ComSoc improves prediction accuracy over single network and naively combined network models by up to 3%. A distributed MapReduce implementation enables efficient inference at large scale.