Fairly abstract - Facebook Engineering - https://m.facebook.com/nt/screen/?params=%7B%22note_id%22%3A10158791436142200%7D&path=%2Fnotes%2Fnote%2F&_rdr

My bad - might be best to ignore the above post as it is a unconfirmed/undated post-mortem that may reference a different event.

Also, Cloudflare’s take on the outage - https://blog.cloudflare.com/october-2021-facebook-outage/

The Cloudflare writeup looks quite solid (loss of network -> no DNS servers -> major issue)