When you create Facebook Custom Audiences using Pixel events (like people who SCROLLED 50 PERCENT or were ACTIVE 30 SECONDS, you need to select the correct event inside Facebook.
This post shows how to select the Facebook Pixel event to create your Audience.
In Facebook Ads Audience Manager:
1. Select the blue Create Audience button, then Custom Audience
2. Select Website Traffic:
3. Then in the dialog, here is where you should find the event:
Click on the All website visitors dropdown, and then you should see a section From your events (see image below).
This list should contain your SCROLLED 50 PERCENT event.
4. You should also be able to find the event by typing into the search box (see image below).
Now you have selected your event, continue creating Custom Audience as normal.
If you select the event without restriction then the audience will be for all pages.
Or, to create an audience of visitors who took a specific event action on a specific page (or pages) you can select to Include More and then specify your Page URL(s).