The program, which was launched on Tuesday in an initial test phase, requires users to pay for the movies using Facebook's on-site currency, called Credits.

The films will be available on their individual Facebook Pages, starting with a Batman film, which costs US$3 to rent.

More titles will be added over the coming months. The service is available only to U.S. users.

With this move, Warner Bros. adds a major digital distribution channel for its movies, while Facebook gets into the increasingly popular market for online streaming of professionally produced films and TV shows. This puts Facebook in competition with the likes of Apple, Amazon.com and Netflix.

In addition, letting its extremely hooked and loyal users watch full-length movies on the site is bound to boost Facebook's already robust usage metrics. According to comScore, in December Facebook unique visitors for the month grew 38 percent year on year to 153.9 million in the U.S., while total time spent on the site spiked 79 percent to almost 50 billion minutes.