with new link_to_app

fb-comments.rb

# app_helper
    def callback_attribute
      if request.env['PATH_INFO']
        " callbackurl='#{link_to_app(request.env['PATH_INFO'])}?_method=GET' "
      end
    end

# in the view
<fb:comments xid="m_story_<%= @first.id %>_comments" canpost="true" candelete="false" showform="true" numposts="5" publish_feed="false" simple="true" reverse="true" returnurl="<%= link_to_app "item/#{@first.id}" %>" <%= callback_attribute %>>
  <fb:title> <%= @first.couple_name %></fb:title>
</fb:comments>