I'm not sure either, but here's some data:

Employees: 350 (Source: Forbes, Dec 15th, 2010) Avg salary, lets assume 80K. (probably too low) TCO per employee $110K

$110K * 350 = $38.5 Million per year on employees.

They've been paying NTT for managed hosting thus far - so much that NTT has cited it as a reason they're expanding their hosting network. They're about to open their own data center in Sacramento - probably because they're realizing how expensive NTT is. My guess is they could be spending as much as $30 million per year on hosting and infrastructure.

Add another $10 for office, admin and legal and you've got a burn rate of just under $80 million per year or $400 million over 5 years.

The WTF here is that they have 350 employees.

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.

According to their COO as of the end of August this year they're still not profitable http://money.cnn.com/video/technology/2010/08/24/tt_twitter_...