Facebook announced a major change today to the proprietary language that all Facebook apps are written in; it will now be made extensible with custom tags that can be shared across applications. The feature will initially be available only on site but will eventually be rolled out to all Facebook Connect supporting sites around the web. We're excited about it but wonder how open it will truly be.

FBML, or Facebook Markup Language as it's called, was intended to ensure that malicious apps couldn't inject nasty code into the browsers of users. We assume that the new markup will have security taken care of by server side processing and this could enable an explosion in feature sharing and code efficiency.

Showcase examples at launch of the feature include:

These tags should be reusable and offer some amount of customization, but the source code doesn't appear to be open. We'd love to see these tags expose their code to developers so they can be more easily altered for reuse in different ways by different apps.

Though there have been thousands upon thousands of apps built on the Facebook platform, many of them are of poor quality and have suffered since the company instituted a site redesign for quality control. Though effective app design is a competitive advantage for numerous apps that are making a shocking amount of money from ads and micropayments, we believe that at least some developers would be happy to share their best practices in the form of open, editable tag code if they were given the opportunity.

We'll see if that happens or if Facebook remains an essentially closed ecosystem, despite this big move made today. Read more about it on the Facebook Developers' Blog.