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Actually i believe its only from the "www" subdomain. Take a look at the robots without the "www"

https://www.twitter.com/robots.txt https://twitter.com/robots.txt

This is likely just to prevent content duplication/nudge users to visit without the "www".

yep makes a huge difference... as explained here as well http://webmarketingschool.com/no-twitter-did-not-just-de-ind...

You should never use Google's estimate as a real estimate, especially once it gets past 100. There are better ways to move content, like 301 redirects or rel=canonical.

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