Buyers have access to a detailed audit trail of their ads and campaign activities. They see where ads they bought ran, the number of people who were able to see their ads, and clicks and other interactions with their ads.

If "these ads are working" is defined as "these ads are accomplishing what they were intended to when purchased," peoples' spend is founded more in data than belief.

Buyers have access to ~2BN people and tools to target them by age, interest, and geography -- much of that demographic and psychographic data is explicitly and freely given by those people themselves; tons more are derived and inferred. Buyers repeat their buys because these ads accomplish what they intended at purchase -- they can effectively drive traffic or commerce or whatever else.

This real power to generate revenue makes the issues around privacy, security, transparency, awareness, responsibility, ethics, laws, oversight more important than if it was created by ignorant beliefs. The latter would be temporary; the former generally strengthens as more data gets into the system.

Every so often there is a post that makes it to the top of HN about how Facebook ads aren't worth the money at all. And that other ads aren't much better. Marketing departments are completely insane (look at the leaked pepsi logo manual) and exist only to continue their own existence. Many corporations don't even bother to test their ad campaigns to make sure they are effective.

The fact that people buy ads is not at all evidence that they are effective.

>It is not a prove per se these ads really are worth their costs.

So you think that Facebook has collectively convinced thousands of businesses who are repeat customers to use something that is losing them money?

Isn't that what advertising and marketing companies managed to do decades before the internet ?