I can see how it can happen. I've worked at some media companies that do a lot less than twitter does and they've hit 120+ employees, just doing ads. Twitter does ads, as far as I can tell via promoted search results, trending topics, and follow suggestions.
There are people who traffic the ads, people who manage accounts, sales staff that get the accounts, marketers, biz dev, product managers, financial analysts, accountants, and engineering + QA who just do ads, optimization, billing, reporting, and trafficking UI. On top of that you have middle management, then senior management on top of that. Add to that support like HR, legal, facilities...it all adds up.
Not saying that it needs to be that way, just that I've seen it be that way.
Generally most of those people aren't making developer salaries, though -- 200k is high for support staff loaded salaries. Account reps, if you don't include commissions, are probably more like $30-50k base.
