What is the ruling of Islamic law on the methods used by intermediaries to calculate the cost of e-marketing, which include: profiting from exchange rate differences, taking a fixed percentage of the advertising value, or selling a specific number of guaranteed views?

The methods mentioned are not free from شرعي (Sharia-compliant) prohibitions and should therefore be avoided. Instead, an agreement can be made with the client on a fixed amount for the service you provide. This amount would be in exchange for preparing and displaying the advertisement on Facebook. There is no harm in whatever you both agree upon, as this method is safer, being free from usury (riba), uncertainty (gharar), or ambiguity (jahala) in the compensation.