I was having a discussion with a fifty-something about Twitter and I was annoyed with his overenthusiasm for Twitter. I'm sure Twitter has a niche market but it will not ever be as remotely as relevant as Facebook.
Facebook has the core features of Myspace and Twitter, so there is no reason why a person would migrate from Facebook to Twitter. The biggest problem I see with Twitter's long term growth is that there are no clear customers. Facebook's friend feed is already superior to Twitter's. Celebrity twitters are also an unreliable source of content. Take a look at Ashton’s threat to leave twitter.
I can't wait for the day that I will no longer hear about Twitter.