Does anybody know of a web service that lets me import my email address book to find my friends on social networks such as Twitter, Flickr and Pinterest?
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Twitter has a built-in feature that let's you import your contacts. See the support page.
The direct link is: https://twitter.com/#!/who_to_follow/import
So does Flickr. See here: http://www.flickr.com/import/people/
According to this blog post, Pinterest has this feature as well. (But I'm not a member, so I can't say for sure.)
I know that LinkedIn, Google+, and Twitter will ask for access to your address book to find people. They may require you to register first.