Here's how to get the Facebook FBML to render correctly in a Ruby on Rails application using HAML markup. Note this syntax is for Ruby 1.9.2 where the hash syntax uses {width: '178px'} in lieu of {:width => '178px'}. Both work, pick what works best for your situation.
Normal facebook FBML code:
<fb:login-button perms="<%= FB_PERMS %>" show-faces="true" width="178px"></fb:login-button>
Correct HAML / Ruby 1.9.2 code:
%fb:login-button(show-faces=true){perms: FB_PERMS, width:'178px'}