Modernizing the Facebook Groups Search to Unlock the Power of Community Knowledge

This post details the modernization of Facebook Groups Search by adopting a hybrid retrieval architecture that blends lexical (Unicorn inverted index) and semantic (Faiss vector index powered by a 12-layer, 200-million-parameter model) search pathways. It introduces a Multi-Task Multi-Label (MTML) ranking architecture to jointly optimize for clicks, shares, and comments. The post also describes an automated offline evaluation framework using Llama 3 to assess search result quality, leading to measurable improvements in search engagement.