What is Facebook Billing Threshold?

As you run ads on Facebook, you'll accrue ad costs. A billing threshold is an amount that you can spend on ads before we charge you. Whenever your ad costs reach your billing threshold amount, we charge you for that amount. It determines when you're charged for your ads based on the amount you spend.

When you first start advertising on Facebook, your billing threshold will be automatically set to a small amount. As you make successful payments, your balance will be cleared. The billing threshold may be raised until your account has reached a final threshold amount. You can think of your billing threshold as a way to build up a good payment history with Facebook.

As an example, let's say that you've just created your first ad and your ad account has a billing threshold of USD 25. As your ad runs, it accrues costs. If your outstanding ad costs reach USD 25, we charge you USD 25.

Once your payment has gone through and your balance has been cleared, your billing threshold may be raised to a new, higher amount. You'll start accruing costs again as your ad continues to run.

So, in a monthly billing period, you could reach your billing threshold once, multiple times or not at all, depending on what your billing threshold is and how much money you're spending on ads. This is why you may be charged multiple times in a given month even if you’re running just one ad. It's also why you may be charged after you purchase ads or even after you've stopped running them.

Source: Visit https://www.facebook.com/business/help/ to learn more.