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New rankings for the U.S. online video market, released by comScore Networks this morning, come with a surprise: has surpassed Hulu in April with more than 30 million unique viewers in the U.S.

We’ve been digging back through comScore’s archive, and it’s the first time that Amazon has topped Hulu by that measure.

It’s a good sign for Jeff Bezos & Co. as they attempt to build their online video business. The growth in viewership follows the launch last fall of the Kindle Fire tablet, which represents a significant new platform for the company’s Instant Video service.

Amazon has also been beefing up the catalog of shows and movies available at no extra charge to people who subscribe to the $79/year Amazon Prime subscription service.

That said, Amazon still has plenty of room for growth. Amazon is No. 8 in the online video rankings, behind giants including Google, Yahoo, Facebook and Microsoft. (Netflix traditionally doesn’t appear in these rankings; we’ve asked a comScore representative for clarification on that.)

Hulu still tops the charts when it comes to total video ads delivered — nearly 1.6 billion in April alone. Hulu stands out when it comes to engaging its audience, with viewers averaging 228.5 minutes on the site, compared with only 17.4 minutes on average for video viewers on Amazon sites.

Here’s the comScore chart showing the rankings for April.

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