Why is Facebook charging my Apple Account?

First see if you can recognise the charge:

How to see what you were charged for

1. To see your purchase history, go to reportaproblem.apple.com.
2. Sign in with your Apple Account.
3. A list of purchases appears. To see family members' purchases, select the Apple Account button and choose a family member.
4. To see the full receipt including tax, tap or click the date. Multiple purchases, including subscriptions, might be grouped onto one charge.

If the charge is for a subscription that you no longer want, you can cancel the subscription. Find out where else you can view your Purchase History.

If you checked reportaproblem.apple.com and your emailed receipts and you're still not sure why you were charged, contact Apple Support.

Looks like a Facebook Page, you probably subscribed to unknowingly.

You would need to find the subscription in you Facebook settings and cancel it.

See the following link for help:

https://www.facebook.com/help/iphone-app/203354304659155