Charities are normally charged zero-rate VAT on advertising they take out in media directed at the general public: radio, TV, newspapers, but targeted mail shots by post or email are standard rated. The VAT treatment of online advertising has now been clarified in R&C Brief 13/2020.
Targeted advertising which is directed at specific individuals according to data the website has collected about them as a subscriber or member, is still standard rated. This would apply to Facebook advertising which is directed at individuals who have logged into the site and have given permission for targeted adverts.
Advertising which is more broadly based, but targeted according to what other websites a person has visited, demographics of the reader, time of day, or what content appears next to the advert, is zero-rated. A full list of the conditions is found in R&C brief 13/2020.
If you act for a charity which has been charged standard rate VAT on advertising that should have been zero-rated, the charity can ask for the VAT to be refunded by the supplier.
Where you act for the supplier of the advertising, such as an online publisher, that organisation should correct its VAT returns for the last four years and claim a refund from HMRC of the VAT overpaid (see VAT notice 700/45).
Where the advert is zero-rated, all of the services connected to that advert, such as design and production should also be zero-rated, so a correction will have to be made by those organisations as well.