In case you’re working through Developing Facebook Platform Applications with Rails, you might have run into problems with authenticity tokens and FBML forms.  Quick dumb fix to save you 2 seconds:

<fb:fbml>
  <fb:request-form
    action="<%= new_invitation_path %>"
    method="POST"
    invite="true"
    type="Karate Poke"
    content="YAYS I added this facebbok app">
    <fb:multi-friend-selector
      showborder="false"
      actiontext="Invite your friends to use Karate Poke"
    />
    <%= hidden_field_tag :authenticity_token, form_authenticity_token %>
  </fb:request-form>
</fb:fbml>