Hello, 

I am trying to use the facebook_uid signup liquid variable in a sign up form, but get the following error:

Liquid error: facebook_uid is not a valid form field for Signup.

My code is:

<div class="span6"><label for="signup_twitter_id">Facebook Handle</label>{% text_field "facebook_uid", class:"text" %}</div>

It should work according to the documentation.