I speculate that it must feel ok to keep hiring when you're using other peoples money to hire them.

Reminiscent of how Digg hired far too many people. When you're taking big money and the press puts you in the same sentence as Facebook there must be pressure to be like the other big companies.

I think there's also an element of betting going on - this is a venture backed startup after all - if Twitter really starts to unlock massive growth over the next year or so then having so many employees will seem a prescient move.