The VAT treatment of Facebooks ads is similar to that of some other common charges for an e-commerce business in the UK. These include Shopify payments fees, Stripe fees, Google Ad fees and the majority of Amazon fees (the key exception to this rule being Amazon advertising fees). The reason for the VAT treatment of each of these fees being consistent is that each of these are services billed to your UK business from outside of the UK, thus being treated under the reverse charge mechanism for UK VAT registered businesses.