http://drupal.org/node/288721 says,

Which version of Drupal for Facebook should I use?
Version 1.x is against the older Facebook client APIs. In other words, it does not include anything for the "new profile", currently found at http://www.new.facebook.com. Unless you're absolutely sure you want to use this, skip it. It will soon be deprecated in favor of the newer Facebook API.
Version 2.x requires the newer API. There are important changes to feed items and the "add metaphor". At the time of writing, this version is under development. Still, it is probably the best version to use.
Checking out of CVS? Here's a guide to the branches...

HEAD (Version 2.x for Drupal 5.x, currently under development)
DRUPAL-5 (Version 1.x for Drupal 5.x. The closest we have to a stable release. Soon will be deprecated in favor of version 2.x)

I was wondering this question myself this morning, but betw the 2.x and 3.x versions. Maybe this FAQ answer could be given an update?