Not so long ago, Facebook released a public API and developers went crazy, marketers drooled, anyone with any interest in reaching the youth demographic started to rethink about how they can bring their products through this channel. A new type of web application was now available, a plugin to your existing Facebook account.
Facebook controls the language, their own proprietary/new syntax FBML, they tended to break deployed code with their security/feature improvements, they control which applications users can browse and see/add. Worst of all, your 'Facebook applications' code could not be re-used easily.
Developers buy some coffee beans and grab a few cases of your favorite caffeinated soda -- It's going to be a wild next few months.