Creating custom audience is use for re-targeting.
This can’t be done unless you already have a list of your audience like emails, ID’s, or phone number that match people in Facebook.
Inside your Ads Manager click “Audiences”
Then click Create a Custom Audience
Click Customer list
Select which one’s for you, as for me I’m uploading my emails list so I select the first one.
File format should be .CSV or .TXT format
Click the Upload File button
Then click Create Audience
In here you can edit the file by clicking to it or just click Next
And just click Done
And you will see this
It usually take about 10 mins for Facebook to make it ready and available.
Your list should be 100.
And there you have your custom audience that you can use to re-target your audience.