tl;dr:Instagram has introduced a multi-stage approach to ranking, including retrieval, first-stage ranking, second-stage ranking, and final re-ranking. The system leverages caching and pre-computation with a Two Towers neural network, making it more flexible and scalable. Techniques like Two Tower retrieval, user interactions history, and parameters tuning - including Bayesian optimization and offline tuning - are employed. The article emphasizes the clever use of caching and pre-computation allowing for heavier models in ranking stages, and concludes with a note on the ongoing complexity and future improvements.