Before we jump the gun on this "loss", read this first:[1]

  Net loss - GAAP net loss was $511 million for the fourth quarter of 2013 compared to a net loss of $9 million in the same period last year. The company's Q4 GAAP net loss included $521 million of stock-based compensation expense, of which $406 million was for restricted stock units previously granted to employees, for which no expense had been recognized, until the effective date of our initial public offering in accordance with GAAP.

  Adjusted EBITDA - Adjusted EBITDA was $45 million for the fourth quarter of 2013 compared to $18 million in the same period last year.

  Non-GAAP net income / loss - Non-GAAP net income was $10 million for the fourth quarter of 2013 compared to a Non-GAAP net loss of $0.3 million in the same period last year.

Oddly enough the "The company's Q4 GAAP net loss included $521 million of stock-based compensation expense, of which $406 million was for restricted stock units previously granted to employees, for which no expense had been recognized, until the effective date of our initial public offering in accordance with GAAP." seems be tucked away in "Research and Development" part of the P&L.

It's worth noting they have $2.2b in cash:

> Cash, cash equivalents and marketable securities - As of December 31, 2013, cash, cash equivalents and marketable securities were approximately $2.2 billion, compared to $321 million as of September 30, 2013.

TL;DR - This $645m "loss" isn't what you think it is.

[1]https://investor.twitterinc.com/releasedetail.cfm?ReleaseID=...