Facebook is now letting third-party application developers create their own snippets of code, called “custom tags.” These tags can be used to make an application perform better as well to build features from one application into another application. For example, a game app could use these tags to integrate a song from a music application.

Facebook also plans to further develop these tags for Facebook Connect, a service that lets third-party sites do things like let users sign in and comment using their Facebook identities. One day, these sites could embed Facebook applications as widgets, making the sites more valuable using the social identity of Facebook users.

Here’s an example from Facebook about how these tags might be used:

So if you’re an application developer and want to make your application even more content rich — say by including music or news — consider using an available custom tag set. For example, if you’re building a wedding site and want to include a playlist of top wedding songs, you can use the iLike custom tag to embed a playlist of top wedding songs. See below for more information about iLike’s custom tags and other examples.

The custom tags make use of Facebook’s existing proprietary markup language for app development, called FBML — a simple format that the thousands of Facebook app developers are already familiar with. For developers, here’s a custom tags directory that Facebook has created to help get you started.

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