> And I'm sure the real expenses of Twitter are all about managing the nine billion pies its got its fingers in, especially the massive hassles of moderation and placing advertisements.

Exactly.

> Still... it feels like it just shouldn't be anywhere near that expensive to fulfill the core, very simple purpose of Twitter.

Fully interactive, user-generated content, number 4 website in the world by traffic... [0]

At this scale people have issues like physical availability for inter-dc data transfer and site-reliability [1]. You are probably directly dealing with (several, since not a single one will be able to fulfill one whole order)OEMs to design server racks to your specifications. [2]

[0] https://en.wikipedia.org/wiki/List_of_most_visited_websites

[1] https://blog.twitter.com/engineering/en_us/topics/infrastruc...

[2] https://www.opencompute.org/documents/ocp-yosemite-v3-platfo...