I can't fathom why they need so many employees. Do they do anything else than the Twitter app? why the **** would you need over 5,000 people?
It's not just the Apps that needs to be built. The harder part is building a performant, scalable, highly available globally distributed back-end. That requires a huge number of staff to build, maintain, monitor and respond to production issues.
There's a few hundred million people using Twitter every day... that's a lot of data to move around.