Twitter has two problems, perhaps three that make me think it might be the technological equivalent of pogs. First, the service has no natural revenue model. Where can you put in an ad in 140 characters? I suppose you can add them to feed pages, but tweets are already freely syndicated. Reining that back in would be really hard. It is reported that Twitter has a revenue model in place. Interestingly, the company is tossing around whether to get bought first based on others' pie in the sky assumptions about the service or turn on the revenue stream and risk having to be judged on actual performance. The waves that revenue will likely send through the Twitter stream are inevitably going to (a) turn off some and (b) cause enough turmoil to make potential suitors wring their hands.
Second, Twitter is the "League of Extraordinary Gentleman" of technologies. The superhero flick starred Sean Connery who, when asked why signed on to the colossal bomb, said that superhero films were really in and he wanted to get in on it. The movie picked superheros nobody cared about, was made by people who didn't understand the comic/superhero culture that attracted young audiences, and thus failed with the only demographic that mattered.
Blogging and social networking technologies have generally been bottom-up endeavors. MySpace and FaceBook had a young, committed base of early adopters and then grew into corporate behemoths. Blogs ran the same route in the publishing space. Twitter bypassed that as media outlets like CNN and ESPN, desperate not to miss the next big thing, dove headlong into Twitter. The result is that Twitter's user base is predominantly 34+. More importantly, I think it is made up of people who feel they need to Tweet to keep up, not because they like it. Consequently, if popular opinion turns against tweets, I would expect usage to crash.
Third, Twitter is a technology that doesn't work as intended. The platform was created to broadcast tweets from one phone to many. Now the whole thing is online, where the space and text-only constraints of SMS make little sense. Even those using twitter on mobile devices are using it in apps or browsers, not in SMS. Why are we shoe-horning the SMS experience on the web, or vice versa.
I have been looking around for that thing in the first decade of 2000 that we'll look back on in 10 years and think, "What were we thinking?" More and more I think Twitter is that thing.