Why Facebook services were down?

A search for "Facebook"[3] here on HN gives lots of hits. here are some of the highlights:

https://en.wikipedia.org/wiki/2021_Facebook_outage[0]

https://blog.cloudflare.com/october-2021-facebook-outage/[1]

https://riskledger.com/blog/facebook-outage[2]

[0] https://news.ycombinator.com/item?id=28752131

[1] https://news.ycombinator.com/item?id=28752131

[2] https://news.ycombinator.com/item?id=28752318

[3] https://hn.algolia.com/?dateRange=all&query=facebook&sort=by...