Twitter has around 8,000 employees. I fully understand and respect the high level of technical skills needed to operate and manage such a highly available service, BUT I still can't for the life of me figure what the other 7,950 employees do.
First, it's a complex product. Just for the user-facing side of things, there are four big codebases: back-end, iPhone, Android, and Web. The back-end stuff is very large, generally for good reason, and sometimes not. A lot of those good reasons relate to performance; with over 100m people using it every day, getting the right tweets to the right people is challenging, especially given that some people have over 100m followers and many users will follow a lot of accounts (I follow over 3300, for example).
But the user-facing side is only part of it. A big way the site makes money is ads. This is in some ways a more complex problem than the user side of things. If you'd like to see, try buying an ad on Twitter. They also have a division that does data products, including a variety of APIs, and another group that has other products for businesses, like tooling for the customer support interactions that people expect to handle on Twitter.
I of course can't forget their SRE folks, who keep all of the machinery humming and make sure all the software is doing what it's supposed to. There a quite a lot of people doing infrastructural work making tools and products that you never hear about and I probably can't list. There's also a good developer tooling group that helps keep the developers above working smoothly. And let's not forget their internal IT group,
Adjacent to engineering are a lot of really sharp product and design people, as well as a design research crew that understands the many ways people use Twitter and examine how it works for them.
We then must turn to people who handle the social side of this. When I worked there, that included a significant staff doing policy work, trust and safety enforcement, and handling darker things like CSAM and terrorism issues. There were also a bunch of people fighting the banal spammers.
And let's not forget the ML people! That alone was a few hundred people doing research, creating and improving models, and applying them to many of the things I mentioned above.
And at last we come to the kind of things that pretty much any large company needs: finance, legal, marketing, government relations, sales, computer security, physical security, admin, management, and the like.
If you ever wonder this in the future about any company, I suggest you look at their jobs site [2], which is always a good way to get an idea of what goes on at a company.