What is this method? Target Lookalike Audiences on Facebook.
Why? By creating lookalike audiences, you are telling facebook about the people who have bough from you and then facebook will find potential new customers that share those same characteristics.
You can also use tracking pixels and data from app installations to help you create lookalike audiences. This is an excellent way to make the data on your existing customers work for you, as it effectively allows you to greatly expand your reach with minimal effort and use highly targeted ads to entice Facebook users who are very similar to your existing customers.
You of course need to have a facebook ad account: https://www.facebook.com/business/
Once you have an ad account, make sure you have the pixel setup as well: https://help.shopify.com/en/manual/promoting-marketing/analyze-marketing/facebook-pixel