And see what? Mostly 1-to-1 events, no search, no broadcast, no history, not badgered with legal matters on any comparable scale, ...

I don't think it's comparable.

Legal would be outsourced, marketing same, spam control - yup, the same. That leaves developers and critical function staff. Twitter could easily work with 200 staff from 2 locations.

I'm curious: do you have experience running an organization like this, or are you speculating?

Working for. I was working for an outsourcing company that catered for companies like that. My hourly rate were ridiculously high, but still companies would be better off hiring us, than employing.