But there's an important number missing from Swisher's post and all other Twitter punditry: How much does it actually cost to run Twitter, and how will that number grow ever more rapidly as more customers sign up?

Twitter's PR team hasn't responded to my email, so I'm stuck with guesses. First, how many employees does Twitter have? says 31. Using a West Coast average of US$125,000 per year in salary, benefits and overhead for staff, that's just under $4 million per year for staff.

More importantly, how many tweets per month is Twitter sending to customers, and how much does it cost them?

Twitter doesn't charge its users anything. The company pays for all messages sent to cellphones through Twitter. That's the burn that no one's been able to estimate with any certainty.

Last year, the claimed costs of $1,000 per user per year to send SMS messages to Europe -- which is why Twitter stopped sending them. But there are no numbers for what its U.S. customers cost. To calculate that, we'd need to know the total number of tweets sent through the system in, say, a month, plus the per-message price that Twitter has (hopefully) negotiated with cellphone carriers. I can't even find a good guess anywhere on either of those.