Amazon is posting their deals on twitter. I'm not quite sure that people would want deal ads on their cell phones all the time...
I'm kind of amazed, as are other people in the naysayer category have been, that Twitter had taken off as it has. At its basic form, it's just passing back and forth messages, a problem seemingly solved by email decades ago. However, twitter obvious is not a question of the underlying technology, but rather, how it is presented to and used by people. It's gotten people use to the idea of instant self-expression, no matter how inane--for better or worse. I would have chalked it up for sensors to monitor and log ourselves, but twitter demonstrated that people will report or say anything if there's an audience. Perhaps trolls have already paved the way in this regard.
That said, I think it's easy to write Twitter off as a fad, since the world's largest collection of quips doesn't quite seem to make the world a better place. My guess is that there's probably value in Twitter, but only when it's married with other sorts of data or text processing. Just off the top of my head, geospatial data and emotion detection algorithm on twitter data could generate a heat map of how people are feeling place to place, or time to time. I imagine advertisers would find this information valuable, since they can set up targeted advertising when people are statistically most vulnerable to impulse buying at a certain time or place.
If twitter can manage an API or platform to support this sort of thing, they'll be around for a while, I think. If not, well, at least we'd have the largest collection of quips for the archaeologists of the 22nd century.