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.