Modules/fb has a long (sometimes arduous) history. Integrating Drupal
with Facebook's platform basically since the platform became
available. Back then, all Facebook apps were what we now call "canvas
pages"; Facebook Connect did not exist. Canvas pages were written in
FBML, whereas now they are iframes. Facebook has completely changed
their PHP and Javascript SDKs, and introduced the "Open Graph". In
short, these modules have changed a lot over the last couple years.
Almost every feature has been reimplemented once or twice.

As with most software projects as they get older, it's tempting to do
a complete rewrite. Doing so would clean out the obsolete bits of
code that are no longer used, or no longer best practice (or no longer
working at all). Here are some thoughts about what this
rewrite-from-scratch might look like.

• No Dependency on PHP SDK -- Fortunately Facebook's platform has
  evolved to be based entirely on internet standards. Most
  interaction is based on JSON over HTTP. Drupal already has helper
  functions for these protocols, which modules/fb can use instead of
  what facebook provides. Frankly I expect Drupal's code to be more
  reliable, better implemented and supported. Plus the modules would
  be self-contained, not requiring an extra download as part of the
  install process.
• Less Canvas Page Code -- Initially, app were all canvas page, and
  canvas pages were difficult to support. These days, they are
  iframes, which makes them basically a regular web page. They are
  not restricted to just 760px. Drupal already does a good job
  serving them up. It might be possible to eliminate fb_canvas.module
  entirely.
• Less Application Configuration -- Once upon a time, modules/fb could
  create an app for you. You never had to visit facebook.com, cut and
  paste IDs and SECRETs etc. That's no longer possible, and facebook
  has a bunch of properties associated with apps that Drupal cannot
  modify. In short, you basically have to visit facebook.com to
  configure your app. And the tools provided by facebook are much
  better than they used to be. So, modules/fb should work well with
  apps that are not configured via Drupal. There are some features,
  like posting to facebook, which do not even require that an
  application is hosted on Drupal.
• Token Management -- These days, interaction to the facebook platform
  is all about tokens. There's a token for each user (or page) that
  uses an app. These tokens make features possible, even if Drupal is
  not configured with an application secret. So, as features become
  more dependent on tokens, modules/fb has to provide a clean and easy
  way to manage lots of tokens.

These are changes I have in mind for a next release (modules/fb 4.x).
The goal being a cleaner, smaller set of code. Not supporting all the
same features, its worth noting. I think its best to do away with the
parts that may no longer be needed, don't work optimally and and hard
to support. Even if doing so will lead to some headaches for users
upgrading from the current version. That is, after all, the Drupal
Way.