Facebook is now offering the Batman film "The Dark Knight" as an experimental video-on-demand movie rental, via an app created by Warner Bros. It's a small move one little film, already available everywhere else, for about the same $3 rental fee but it has big ramifications.

It could pave the way for Netflix and Hulu to build apps that fit the social network, or it could further alienate them, another giant moving into their space. Speaking of giants, it is certainly a threat to Apple, Google, Amazon and Microsoft, each trying to reach home and mobile audiences with its own video approach. Since the $3 rental fee is taken in the form of 300 Facebook Credits, you can bet that whatever happens, Facebook is quite interested in getting a piece of the action beyond just advertising.