[carlosjobim]

> It's impossible to verify

Spend on ads is extremely easy to verify. For every ad channel you use a different discount code. Then you see how many customers used a specific discount code when making a purchase. Discount codes work everywhere: Social media, search ads, radio, TV, banners, etc etc

If you don't use discount codes when advertising it means you don't care for how you spend your marketing money - which is the truth for a large majority of businesses. They simply don't care if their advertising works or not.