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That's an interesting angle and I always thought, why is there a search.twitter.com and not just a little input at the top every page on twitter.com.

I suspect that that design decision is failwhale repellent. The only problem with a search box on every page is that people would probably use it.

IIRC, twitter not too long ago acquired a 3rd party company who offered better searching. It may be an integration issue as well.