On the company's earnings call Monday, CEO Dick Costolo described the varying types of Twitter users as a series of concentric circles. At the core are the signed-in users who tweet regularly and operate as the "fuel that power's [Twitter's] entire system." But the next circle out from this core consists of Twitter's signed-out users who visit the site regularly without ever registering an account. Twitter can't seem to convince these people to register accounts, which is essential for selling quality targeted ads against them.