  After the success the RecSys 2020 Challenge, we are describing a novel and
bigger dataset that was released in conjunction with the ACM 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 into consideration fairness aspects
of the challenge. Unlike many static datsets, a lot of effort went into making
sure that the dataset was synced 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, highlighting some of the
issues that arise when creating recommender systems at Twitter scale.
