Instagram’s CEO has said one of the most valuable parts of being acquired by FB is they could just hook into their vast revenue org (probably thousands of employees, and battle tested ad targeting tech and expertise). Twitter has to build its own, and I’m sure that accounts for most of their headcount.
For a while stories popped up describing Devs lounging around or playing table tennis, waiting for their stock to vest. I've no idea the truth in these anecdotes and they're annoyingly hard to find due to google finding lots of tweets instead of articles about twitter.
You can add -site:twitter.com to remove all tweets from your search results when you're trying to find something about twitter instead of something on twitter.
I've often wondered how Twitter ended up with so many people as well. I'd love to see a breakdown of what they are all doing.