In this paper, we present NxtPost, a deployed user-to-post content based sequential recommender system for Facebook Groups. Inspired by recent advances in NLP, we have adapted a Transformer based model to the domain of sequential recommendation. We explore causal masked multi-head attention that optimizes both short and long-term user interests. From a user's past activities validated by defined safety process, NxtPost seeks to learn a representation for the user's dynamic content preference and to predict the next post user may be interested in. In contrast to previous Transformer based methods, we do not assume that the recommendable posts have a fixed corpus. Accordingly, we use an external item/token embedding to extend a sequence-based approach to a large vocabulary. We achieve 49% abs. improvement in offline evaluation. As a result of NxtPost deployment, 0.6% more users are meeting new people, engaging with the community, sharing knowledge and getting support. The paper shares our experience in developing a personalized sequential recommender system, lessons deploying the model for cold start users, how to deal with freshness, and tuning strategies to reach higher efficiency in online A/B experiments.

In this paper, we present VisRel, a deployed large-scale media search system that leverages text understanding, media understanding, and multimodal technologies to deliver a modern multimedia search experience. We share our insight on developing image and video understanding models for content retrieval, training efficient and effective media-to-query relevance models, and refining online and offline metrics to measure the success of one of the largest media search databases in the industry. We summarize our learnings gathered from hundreds of A/B test experiments and describe the most effective technical approaches. The techniques presented in this work have contributed 34% (abs.) improvement to media-to-query relevance and 10% improvement to user engagement. We believe that this work can provide practical solutions and insights for engineers who are interested in applying media understanding technologies to empower multimedia search systems that operate at Facebook scale.