They could have probably done both, given what Facebook managed to accomplish financially. FB was profitable at $500-$600 million in sales and around 200 million monthly active users. At that same size Twitter was losing an immense amount of money. At the $2.5 billion sales level that Twitter hit in fiscal 2016, Facebook had around $750-$900 million in net income.
Likely this is true for engineering as well. 25% of the team pulling the weight of the department.
Given that, if you're selling something like Github or container management services, you can imagine that the person who's selling in the Pacific West Coast region might be having an easier time of things than the guy who has the mid-west. This isn't universally true for all products, but sales people will complain about leads being bad in any industry (see: Glengarry Glen Ross).
All of that is to say is that empirically, usually there are a few outperformers on any sales team that are crushing quota, and a bunch of others who are skating by at fractions of quota until they get canned. As some evidence, there's a lot of churn in sales teams and for the average sales person, which is partially because of the above, and also because good sales people will kill it as a company is getting a lot of adoption, and then jump ship when a better opportunity arises.
So maybe there's been a study on this stuff, but you could talk to any number of people who work in sales orgs and hear similar stories without the study.
Oh, and yes, when companies get 'serious' about sales, you'll see sales quickly expand to encompass a third or half the total company size. I won't make any guesses as to whether that actually makes sense and how many people are being productive, but large sales orgs are common in my understanding.
Good sales people bring in enough revenue for a company to be profitable. But I have no idea how many sales people Twitter has, I think even if they had 100 it'd be plenty.
Remember that Twitter works in multiple countries and in different languages. Then there is a lot of face to face meetings required, so add offices and service staff as well.
1-1,5K would sound reasonable for global sales and customer services.
I would assume that it is the lack of products/top commitment that causes poor sales.