Actually... (2019) https://ai.meta.com/blog/dlrm-an-advanced-open-source-deep-l...
Source code:
https://github.com/facebookresearch/dlrm
Paper:
https://arxiv.org/abs/1906.00091
Updated 2023 blog post, but solely for content recommendation, but ads recommendation is ~90% the same:
https://engineering.fb.com/2023/08/09/ml-applications/scalin...
It's a little out of date, but the internal one is built with the same concepts, just more advanced modeling techniques and data.