The massive income Facebook has seems to imply so.
It is not a prove per se these ads really are worth their costs.
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.
The fact that people buy ads is not at all evidence that they are effective.
So you think that Facebook has collectively convinced thousands of businesses who are repeat customers to use something that is losing them money?
They were steadily losing users in the very significant 13-17 years demographic, had a CTR one fifth of the rest of the web. Several reports of brands quitting facebook or advertising on facebook because it is expensive and does not seem to be effective on a background of unstable rules and "quirky" system with some overcharging and ads not displayed.
Facebook built itself on investor story time, with promises of better targeted ads in the future, and up to now is still selling the idea that at some point in the future it will succeed at this. It has yet to deliver on this promise in a consistent way.