The thought that a company like twitter, that has about 5 features anybody cares about (and pretty much had them for years) spends ONE BILLION PER QUARTER on "R&D" and operations boggles my mind. I mean, 250 people or something, I would understand (but only if there is no major outsourcing involved). But THOUSANDS of developers? How? Why?

There certainly was some effort involved to make Twitter performant if someone with 100m followers posted a tweet. But that seems to have been solved for quite some while and since the number of active users is quite stagnant, I don't see any order of magnitude changes coming which would require a lot of "R&D" on that end.