It's interesting, because it's hard to find a reason why this is happening. Twitter has entered pop culture, it's been the web buzzword of the year, everyone from NFL players to rock stars are tweeting. We've seen hundreds of exciting services and ideas built around Twitter in the past two years. Twitter has been adding a lot of new, important features lately, the latest of which has been the retweet function. That's got to count for something, right?
In some ways, Twitter is more mainstream than Facebook. And yet Facebook is growing again (despite its huge audience size), and Twitter is not. You can chalk it up to Miley Cyrus' departure, to some cosmic anomaly, or you could just say that Twitter was always too geeky for the wide mainstream audience. But it's odd for a service to grow so explosively and to be featured in the media so much, and then to stop growing all at a sudden without a clear reason.
I still believe that this won't matter in the long run; Twitter aims to be a ubiquitous service - the new SMS, if you will - that acts almost as a part of the net's infrastructure. It doesn't have to be popular, it merely needs to be there, behind the curtain, sending your tweets back and forth. After all, it has a search deal with both Bing and Google, and that's bound to drive some traffic their way.
However, I'm interested in your theories on why its traffic has stopped growing. Is it just an anomaly? Have users grown tired of it? Has it failed to engage a certain key demographic, for example teens? Please give us your theories in the comments.