Facebook already kicks butt when it comes to “dwell time,” the measure of how long people spend on a particular site or page — an important metric for online advertising. Just imagine what this is going to do on top of that.

The announcement today that Warner Bros. will begin testing movies inside Facebook is a big development in the world of technology and entertainment, positioning the social media giant to become an even bigger player in the digital world. It’s also an interesting expansion of the Facebook Credits system, demonstrating the company’s potential to become a bigger player in e-commerce.

Warner Bros is starting with “The Dark Knight,” which will cost 30 Facebook Credits, or $3, to rent. Rentals will be good for 48 hours, offering features including the ability to resume playing at the same spot when people log back into their Facebook accounts. Users will also be able to interact with their Facebook friends, post comments and update their Facebook status from the movie page.

Following this development to its natural conclusion, it’s not hard to imagine Facebook becoming a major player in the living room someday, far beyond current implementations such as the version of Facebook in Microsoft’s Xbox Live.

Coverage: New York Times, AllThingsD, CNet News.com, msnbc.com.

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