And a good chunk of it is maintaining & improving a large throughput backend system that is one of the top 10 in the world in terms of size and data volume.

And on top of that, an even smaller fraction is the people who think of and implement new front end things that you would interact with and think of the app changing. Like a lot of companies, it's probably a 10:1 ratio as far as backend:frontend people there are at the company.

As to why twitter changes so little, the company itself doesn't really understand why it's successful, so there is a lot of conservatism inside of it to not screw with the golden goose.