begun renting movies on Facebook for 30 Facebook Credits, or $3.
The films are packaged as apps -- like FarmVille and CityVille -- but it really is as simple as heading to a page, using 30 Credits, and watching the movie. You can watch the movie as many times as you like for 48 hours, after which the rental terminates. You can stop and start the movie whenever you like, too.
Facebook Credits, if they somehow passed you by (there must be a few people who don't play FarmVille, right?) are Facebook's new all-encompassing currency. Every virtual good transaction (such as in-app purchases, or renting a movie) must use Facebook Credits. Facebook, like Apple, takes a 30% cut of all purchases with Credits.
For now, the only movie available to rent on Facebook is Batman: The Dark Knight, but more should begin to appear soon. If you 'Like' a movie, you should see a status update when it becomes available to rent (see the Dark Knight page for an example).