There is only one audience more valuable than your current customer: people sharing your content.
They might be your customers, or not, but the fact that they think your content is worth sharing with their friends, that it will make them look good, has a value beyond your current campaign’s goal.
It is free advertising. It is more effective than your paid ad because it is coming from people that the new audience already trust.
There is one simple trick you can do to optimize your ads for sharing: check on which platform/placement people are sharing your ads the most:
Regardless of their demographics, if people are sharing ads placed on Facebook’s newsfeed, and not in the Audience Network or Messenger, then put the majority of your budget there. Remove the platforms that don’t spread your content.