"Initially, FBML included only tags that Facebook created," explains Yariv Sadan at the Facebook Devloper Blog. "With custom tags, any developer can create new FBML tags. Developers can use these tags in their own applications, or they can share their custom tags with the entire Facebook developer community as pre-built FBML components."

In fact, Facebook has a wiki called the Custom Tags Directory for developers to add their apps. So far, it includes entries from applications like iLike, Causes, Graffiti, and Visual Bookshelf.

Custom tags are only available internally within Facebook, but will soon become available for sites integrated with Facebook Connect. Of course questions of security are bound to come up.

"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," writes Marshall Kirkpatrick at Read Write Web. "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."

Assuming that Facebook has thought these things through, users of the social network can probably expect some pretty cool things to come out of this news. Perhaps we’ll see an increase in the usefulness of Facebook apps in general.