Finally, the biggest problem of all is the network effect of Twitter's popularity. As the number of people using a network goes up, the number of connections between them rises much, much faster. Most Twitter users now have several times more followers than they did a year ago. As a result, every update sent to all of a user's followers costs the company more to send to everyone who's subscribed by phone.

It's an : The more people use Twitter, the faster the number of messages grows. Twitter's SMS bill is not only climbing, it's accelerating, and each new customer costs more than the last one. Any company looking to acquire Twitter has done calculations to estimate the future cost of the service, which could grow from a reasonable expense to a profit-destroying nightmare if Twitter's acquirer were successful at marketing the service without charging for it.

Too bad none of those estimates have yet been leaked onto the Internet. Anyone?