I suppose if you're aiming for the next best thing after Twitter, it might be brief videos with voice recognition. eg "Joakal: Met HN's founder!" "(PG talking) I like blackberries. Blackberry companies are best" (10, 20 or 30 secs?). That would make spam control and aggregation pretty challenging but lots of potential for advertisements and interviews.
Here's a brief advertisement (edited), "The Last Exorcism - BEST OF Chatroulette reactions": http://www.youtube.com/watch?v=CNSaurw6E_Q And brief comedy (edited), "Top 20 5 Second Films Presented By UPROXX": http://www.youtube.com/watch?v=saqO_ZqX6uY
The glasses could also detect a user's emotions by their eye state and voice (or pulse.) It may autodetect location and detect sounds to attach environmental metadata -- so great metadata. Think The Terminator combined with Brainstorm.
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.
Make something that makes each one of those "everyone" to leave twitter and join your site and you'll make it irrelevant.