After the success the RecSys 2020 Challenge, we are describing anovel and bigger dataset that was released in conjunction with theACM RecSys Challenge 2021. This year’s dataset is not only bigger(~1B data points, a 5 fold increase), but for the first time it take intoconsideration fairness aspects of the challenge. Unlike many staticdatsets, a lot of effort went into making sure that the dataset wassynced with the Twitter platform: if a user deleted their content,the same content would be promptly removed from the dataset too.In this paper, we introduce the dataset and challenge, highlightingsome of the issues that arise when creating recommender systemsat Twitter scale.