Earlier quoted context omitted.
Twitter was twitter before they had all those people. Sure, but it can't scale without them. How exactly is Twitter supposed to pay the bills? People use this platform because, for the most part, the basic premise is entirely free. How else can they fund the infrastructure/R&D costs?
scaling...it did. That is it had a ton of users using essentially the same product with a much smaller team, at one point. Paying the bills.., IDK. Presumably they've determined that these people are useful in this regard. Otherwise, why have them? But from a first principles reasoning, it seems possible to sell add without thousands of ad sellers.
As an aside it's odd to think of twitter and google etc as a kind of modern successor to the MadMen ad house. I guess if you were being pedantic, they're more like the modern version of what the network TV commercial industry was in the 60's.