[ohashi]

In theory there are only two options for this.

A. Something that is better than Twitter comes along and people like it more.

B. People no longer find Twitter useful/interesting/whatever makes them use it in the first place.

So basically, irrelevancy by competition or by societal shift.

[sp332]

Lots of things are "better" than Twitter, for almost any way you could measure "better". The only thing Twitter has going for it is size - everyone is on Twitter. Also they are now really fast and pretty reliable, but even back when that wasn't true, they were still growing like crazy.

[anujkk]

Well, you can also look it in opposite way. Why is everyone on twitter? There must be something that twitter is doing better than others? Ease of use? simplicity? better marketing? hype? Why are you on twitter?

Make something that makes each one of those "everyone" to leave twitter and join your site and you'll make it irrelevant.