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Advertising: accept payments for interleaved inserts or higher-prominence of 'native' material.

Works for Google, Facebook, Twitter!

Well, they "made" $225 million with them so far ;)

Pinterest hasn't really tried to monetize at all, yet... Wher is this $225 million figure from?

See the title of this post, the $225 million they raised.

I'd wager that social referrals refer to affiliate fees gained through their users buying products shared on the site.