Twitter has a serious duplicate content problem
Twitter seems to have got things the wrong way round when it comes to SEO.
They are blocking the search pages which Google really wants to index and not blocking the pages that really shouldn’t be indexed at all. Currently the api.twitter.com sub-domain has over 2 million pages indexed and the explore.twitter.com sub-domain has over 8 million pages indexed.
It’s not clear how Google is finding these pages, over 10 million isn’t the result of bloggers randomly linking to the wrong things. My guess is a rogue sitemap or RSS application is going wrong.

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May be Twitter does it intentionally because they don’t want to let Google index their search results. May be they want people come and stay on Twitter?
Well, Twitter may be facing this dilemma because of SEO but it may not be the only reason.
Read something similar few weeks back ~ Twitter Domain Duplicate Content Issues – http://hwork.org/2009/03/06/twitter-domain-duplicate-content-issues/