Would you watch movies on Facebook? That’s what Warner Brothers, a division of Time Warner (parent company of Fortune), wants to know by offering up The Dark Knight for viewing on the movie’s Facebook fan page. 3.9 million fans have already “Liked” it and they and other Facebook users can rent it for 48 hours for 30 Facebook Credits, the equivalent of $3. There are some caveats though: it’s only available in the United States, only streams in Standard Definition, and sharing remains limited to posting the activity to the NewsFeed, “Liking,” or Tweeting it. Pretty barebones at this point, considering the wealth of features Facebook currently offers.

But the move is less notable for what it’s lacking and more for what it means for content programming going forward. Recently, Warner Brothers also dabbled with other unorthodox distribution methods, by creating iPad, iPhone, and iPod apps specifically for The Dark Knight and Inception. Not an ideal solution — who wants to download an iOS app for each and every movie? –but it does show that the studio wants to reach as many markets as possible. Its movie apps reach 35 worldwide markets, more than the 23 that iTunes covers. With Facebook, Warner Brothers has the potential to one day reach some 600 million users in more than 200 countries, from Jordan and Senegal to Botswana and Martinique. In doing so, it becomes the first studio to offer movie content within a social network. No outside links or shuttling to third-party web sites.

It might all sound pretty strange at first. Because when users signed up for Facebook, they envisioned using it for connecting with friends and acquaintances, and in some cases, people they’re dying to date. Watching movies probably didn’t register. After all, there are better, dedicated services like Netflix to handle that, right?

But the more you think about it, the more it makes sense. Facebook’s userbase spends an average of 55 minutes a day, 6.5 hours a week, or 1.2 days a month on the site, a significant chunk of time only rivaled by email usage, and one that will grow even further as Facebook matures and introduces more time-sucking features. Rather than force Facebookers to leave the network to some external service or site, a huge barrier of entry in and of itself, content providers are bringing programming to where the users are.

“It’s a great opportunity to bring a shared viewing experience back to folks who have gotten geographically dispersed,” says David Raycroft, VP of Product Strategy for Milyoni, the social commerce platform used for the Dark Knight-Facebook viewing and ecommerce app. Development of such projects can take as little as three days, though in this case, The Dark Knight app took 30 days to develop.

While it’s a solid first step — the user interface is simple and responsive — it also raises several thorny issues. Once the whole trend of Facebook movie streaming takes off, and I firmly believe it will eventually, there’s the issue of fragmentation. Will users be forced to seek out movie fan pages to watch that particular flick? How will they know when a particular new movie becomes available for viewing?

A content streaming hub will become vital as more studios bring content over to the social network, not only to aggregate and highlight content in one central location, but also to update users. Of course, in doing so, it brings Facebook one step closer to being even more integral to your daily life, which, if you’re Mark Zuckerberg, is a good thing.