SAN FRANCISCO (Reuters) - Warner Bros is making some of its films available on Facebook, opening up a new revenue source for the Internet social network and signaling new competition for online entertainment companies. Consumers can pay for the movies using Facebook Credits, a virtual currency so far used mainly in social games on the site, according to Warner Bros, a unit of Time Warner Inc, on Tuesday. Facebook takes 30 percent of the revenue from sales by third parties on the website using Credits. The service will initially be available in the United States, the company said. The streaming of full-length Hollywood films on Facebook creates a new role for the 500-million-plus member social network, which has grown into one of the most powerful Web players by making it easy to share photos, videos and other content with friends.