According to the Twitter Careers page: Software Engineering, Design & UX Research, Sales & Revenue, Product Management, Infrastructure Operations, Media & BizDev, Trust & Safety, User Services, Legal, Finance, Workplace & Admin, Marketing & Communications, HR & Recruiting.

As far as engineering, take a look: https://about.twitter.com/careers/teams/software-engineering

Many companies are much more complex than you might imagine from the outside. Also, a lot of it is meta-headcount: managers, facilities, food service, payroll, benefits, recruiters, lawyers, HR people, etc. all scale at least partially with headcount.

You'll notice many of the jobs are with companies Twitter has acquired or various platform-level projects, like Fabric, Vine, TellApart, Finagle, etc.

Yeah, I'm surprised by that number as well, but then again, Facebook got tons of news coverage yesterday for adding five emoticons (did they actually add them, or were they just flighting it?). Things are strange these days.

You hire managers, when the direct reports increase.

When you are a manger with 5 direct reports, you want to increase the headcount to justify your existence: you go around looking for projects, and direct reports.

And half of its employees are engineers.