title: Using facebook external_id with pixel and custom audience
body: <p>I am wanting to link visitors who have visited my website and then have performed an offline task  up within a facebook audience.
  I have set up facebook pixel to push a <code>external_id</code> when a user visits my site based on the users user_id</p>
  
  <p>e.g </p>
  
  <pre><code>fbq('init', pixel.id, {
          external_id: "35d3652b57be5238df572b6074a7794d966e3be78b19b32f712d060c8da202d6",
  })
  </code></pre>
  
  <p>(<a href="https://developers.facebook.com/docs/marketing-api/server-side-api/parameters/user-data/" rel="nofollow noreferrer">https://developers.facebook.com/docs/marketing-api/server-side-api/parameters/user-data/</a>)</p>
  
  <p>I have then set up a custom audience (Customer List) which is the audience I want to use.</p>
  
  <p>I later call the facebook api to add users to this custom audience based on their external_id</p>
  
  <p><code>{audience_id}/users</code> with schema <code>EXTERN_ID</code> and an array of external id's to add.
  (<a href="https://developers.facebook.com/docs/marketing-api/audiences/guides/custom-audiences/" rel="nofollow noreferrer">https://developers.facebook.com/docs/marketing-api/audiences/guides/custom-audiences/</a>)</p>
  
  <p>Every thing seems to work and I can see in my facebook audience history that 9000 rows were added but the size still says "Fewer than 1,000"</p>
  
  <p>Is there something I am missing here when using external_id?</p>
  
  <p>Thank you</p>