For my case it works the following configuration:
- Add the fb pixel code with tracking ID in index.html
 
<!-- Facebook Pixel Code -->
  <script>
  !function(f,b,e,v,n,t,s){if(f.fbq)return;n=f.fbq=function(){n.callMethod?
  n.callMethod.apply(n,arguments):n.queue.push(arguments)};if(!f._fbq)f._fbq=n;
  n.push=n;n.loaded=!0;n.version='2.0';n.queue=[];t=b.createElement(e);t.async=!0;
  t.src=v;s=b.getElementsByTagName(e)[0];s.parentNode.insertBefore(t,s)}(window,
  document,'script','https://connect.facebook.net/en_US/fbevents.js');
  fbq('init', '<<PIXEL ID>>'); // Insert your pixel ID here.
  </script>
  <noscript><img height="1" width="1" style="display:none"
  src="https://www.facebook.com/tr?id=133095247279894&ev=PageView&noscript=1"
  /></noscript>
  <!-- DO NOT MODIFY -->
  <!-- End Facebook Pixel Code -->
 
 
Inject $window on the state controller
// Registration
angular.module('app.module')
    .controller('LeadsController', ['$scope', '$state', '$window']);
$window.fbq('track', 'InitiateCheckout');   //or Purchase/Leads/CompleteRegistration event
 
It works that way because basically fbq has is binded on window object.
Hope this helps!