Awareness of Twitter is > 90%. But usage is < 10%.
Why the huge gap?
Because @Twitter is #funky for the #mainstream.
But this could/should change as:
-- Mainstream social usage become more sophisticated
-- Twitter evolves
-- Apple (and others) push the platform
Wikipedia seemed exotic and not trustworthy to many people as little as 5 or 6 years ago. In a similar way, I think Twitter still seems exotic and strange to many. But this could change as more graduate into upper division social media.
Is a new killer app needed for Twitter to close the gap? I'd argue Twitter's current role as a "real-time, social, news/info aggregator" is pretty killer, where wisdom of the crowds meets personalization meets messaging/social (digg meets rss feeds meets facebook). But relatively few people outside of the "tech enlightened" know how to use Twitter very well. It's quirky, with its own protocols, and complexities. But some of the peccadillos will fade with familiarity and others will be eliminated with product evolution.
In the future, I expect the Twitter ecosystem to birth a few more killer apps as well.
FB is likely to remian the 800 pound gorilla. But Twitter's still got some interesting runway ahead.
For more on this, here's a great post on the Twitter Paradox by @BrianSolis