How does billing on Facebook work?
Why did Facebook charge my credit card several small charges?
In the beginning, Facebook will attempt to process several small charges to validate and ensure that future costs can be processed through your credit card. Facebook will gradually increase the amount/s it charges and will create a payment threshold. If you meet that payment threshold amount before your bill date, Facebook will immediately charge your card in that amount, and it will also charge any remaining amount on your bill date.
Why did Facebook charge my credit card twice this month?
Facebook can bill you two separate times – first, if you meet your payment threshold before the bill date, and once more on your bill date. If you meet the payment threshold amount before your bill date, Facebook will immediately charge your card in that amount. You will then be charged any remaining amount of ad spend on your bill date.
This link will further explain how Facebook billing works: https://www.facebook.com/business/help/716180208457684?id=1792465934137726