When facebook introduced yet another API last April, they deprecated FBML canvas pages in favor of the iframe flavor. And developers like me breathed a sigh of relief that we'd never have to deal with straight FBML again (not to be confused with XFBML, still in use on both connect and canvas pages).

But alas, facebook has not (yet) gotten rid of FBML for good. There's still the Profile Tab, which is a nice thing for applications to add to facebook Pages. Unfortunately, to support these your app has to produce FBML.

I recommend producing a simple single page of FBML for a profile tab, but link out to canvas pages to support more complicated features. Think of the profile tab as a landing page, and get the user away from there and into your application as soon as possible.

There's some confusing on the issue queue regarding regarding these tabs. And the cryptic error messages delivered by facebook should go away once you're using an FBML theme.

Here's an example, a very simple one, showing what an FBML theme might look like. We're working on better documentation, and I'll link to that when it's available.