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Twitter has finally added the feature they've needed from day one: the ability to import your contacts from another service. For now it is limited to just Gmail, which should apply to a large proportion of the service's early adopter user base. Enter your email address and password to see which of your contacts are on Twitter, or manually invite people to join.
And, no fears, they won't spam your friends and contacts, just let you add the ones who already have accounts.
Twitter also added people search last week.