Twitter has a usage problem. Or at least in the traditional sense of usage.
Looks good, left-to-right, but it's decelerating. (Especially when compared to other social-esque services.)
Twitter may need to do something about those who only want to consume information. Make the service for newbies far more friendlier, and a gateway to Twitter. (Note, Yahoo is doing this via their mobile apps.)
Ad rates are doing well.