Today Facebook will announce the launch of “Custom Tags”, a new feature which enables developers to extend functionality within their applications to other applications on the Facebook platform. The concept is that applications can launch their own version of FBML tags, ultimately enriching the overall feature set that developers have access to within the Facebook platform. Yariv Sadan, the engineer behind the feature, stated that “FBML is great for developing pre-built components [and we wanted to expand that functionality to developers].”

Currently the custom tags are not available on Facebook Connect powered websites, but soon enough they will be. Once extended to external websites with Facebook Connect, we could see a drastic increase in Connect powered widget adoption around the web. While it isn’t significantly easier to embed custom tags instead of simple JavaScript widgets, within the Facebook platform embedding JavaScript can be cumbersome.

As such, the custom tags will help alleviate some of that frustration by helping developers to easily reuse blocks of code and simplify the extension of features to other applications. Will other applications find it useful to embed features from Causes, Visual Bookshelf, iLike, Graffiti, or any other applications currently offering custom tags? I’m not sure but there’s definitely a lot of potential.

Ultimately custom tags provide a simple way for applications to offer small segments of functionality and/or content to other developers. I asked Josh Elman of Facebook why they don’t just act as a broker of cross-application APIs and he said that it was something they might consider but the current purpose was to “extend the user interface.” Some of these applications make sense for offering compartmentalized functionality.

Graffiti for instance could theoretically let other applications draw designs for specific events. For example, imagine if there was an inauguration application that decided to enable individuals to submit drawings about inauguration. Alternatively “Catbook could enable donations for animal causes using the Causes donation widget,” as Josh Elman told AllFacebook. I’m sure we’ll see new types of cross-application functionality as developers take advantage of the new custom tags feature.

I asked Aaron Batalion, Chief Technology Officer of LivingSocial, about his thoughts on Facebook’s new Custom tags feature. Aaron said, “Facebook’s new custom tag support is an awesome step towards bringing fully featured experiences to Facebook Connected sites. Say the NYTimes FB Connected and put a Visual Bookshelf widget on their Bestseller books page, a user could FB Connect, and immediately see socially relevant context from activity generated with Facebook. What friends are reading those books, what they thought, etc. That is going to be AMAZING!.”

If you’d like to find examples of the custom tags, check out the custom tags directory which will be launching today.