Apart from the obvious references to facebooker and rfacebook. Mainly for sign-ons and getting basic profile info
You've named the two big gems for Facebook integration, both have their good and bad points.
If you are looking for a lightweight method of handling logins and basic information, I'd try the Janrain Engage service. For the same development effort of getting Facebook integration going you could get a handful of other services as well. They have a free and a very reasonable (considering the development time it offsets) pay option.
Got an excellent response on Quora
http://www.quora.com/Any-suggestions-for-a-Ruby-on-Rails-plugin-for-the-new-Facebook-API
Depending on which style you like, top 3 from RubyGems list, based on no of downloads as on Aug 3, 2010
http://rubygems.org/search?query...
fb_graph (0.2.1) A Ruby wrapper for Facebook Graph API 1421 downloads http://rubygems.org/gems/fb_graph
koala (0.8.0) Koala is a lightweight, flexible Ruby SDK for Facebook. It allows read/write access to the socia... 1347 downloads http://rubygems.org/gems/koala
fgraph (0.3.2) Ruby Facebook Graph API 771 downloads http://rubygems.org/gems/fgraph
rest-graph A super simple Facebook Open Graph API client 1275 downloads http://rubygems.org/gems/rest-graph