How does a company lose money? They didn't start from nothing--they have money in the bank from equity infusions (e.g.-going public). You lose money if you make less than you spend. That's what happened here.
Edit: Also, these numbers aren't just cash. Revenue is realized when it's earned, not when it's actually paid. So, for example, if Pandora provides $50MM of advertising, and then bills that, it records $50MM of revenue, even if it hasn't received all of that yet. Instead of being counted in cash, it's counted in accounts receivable (commonly abbreviated as A/R). This adheres to GAAP (in the US).
Edit: Also, these numbers aren't just cash. Revenue is realized when it's earned, not when it's actually paid. So, for example, if Pandora provides $50MM of advertising, and then bills that, it records $50MM of revenue, even if it hasn't received all of that yet. Instead of being counted in cash, it's counted in accounts receivable (commonly abbreviated as A/R). This adheres to GAAP (in the US).