Security, abuse / harassment, fraud, community interaction, developer relations, 24/7 maintenance & ops, APIs, marketing & PR, legal, advertising, physical security / cleaning etc (whether outsourced or not it's a cost), secretarial, accounting, human resources & recruiting, sales, engineering, various management and so on.
The notion that you could run a $2.5 to $3 billion sales business with 100 people is a very bad joke. To deliver on that kind of sales level, you need more than a hundred people working in your sales organization alone. There are only a few types of businesses that can operate that thin at that size of sales, most of which are in the financial world.
Twitter doesn't need to cut down to 100 people to become nicely profitable, they need to cut down to 1,000 - 1,500 (while reducing infrastructure costs). Facebook delivers $1.5 million in sales per employee. Twitter's overall business is less complex and easier to operate than Facebook, they could deliver a higher ratio.
It is a function of complexity, but complexity is often necessitated by scale. More users means more problems with scale, which leads to increased complexity to address the problems, which leads to a larger team needed to maintain the software.
That being said, 50 engineers seems enough to me for maintenance.
However, there's a Presidential Debate tonight so you know Twitter's traffic is going to be off the charts.
It does have less features, but many of Twitter's are of dubious value.
I don't think it's comparable.