In its march to become a portal to the entire internet, Facebook has signed a deal with Warner Brothers to test out whether its 600-plus million users will be willing to pay to stream new movies from within the social networking site’s virtual walls.

Warner Brothers will start the trial with The Dark Knight, the 2008 Batman movie from Christoper Nolan, and Facebook users will have to pay 30 Facebook credits (about $3) to watch the movie. To rent the movie, Facebook users in the United States have to visit the movie’s Facebook page, and start watching the movie from there. The rental period...