Twitter has a lot of mindshare (I can't believe how much I see it mentioned on TV or mainstream newspapers). However, in terms of user base they are still pretty small (6 million?). There will be more competitors, especially since making your app work with Twitter is relatively easy.
I think that the reason it's only 6 million is that everyone else is just using facebook. I use twitter/identi.ca to have conversations, ask questions, "microblog" but when my posts get pushed to facebook, my friends have no idea what I'm doing.
In other words, maybe the best way for someone to take down twitter is to create a compelling reason to not use facebook's status feature, and instead use the next new thing.
