So long as the fraud is uniformly distributed, that is true. But more significant is the fact that you really can't estimate your costs and benefits correctly when the vendor (Facebook) is giving you incorrect or fraudulent data.
I agree with what you say about the estimation of costs.
The benefits to site owners normally take place outside of Facebook and so should be measurable regardless of what Facebook do.
In the case where the benefit is within Facebook then the buyer is purchasing likes or similar and then they are in for a world of hurt (in my experience) because of the difficulty in valuing what is being bought.
In this case the advertiser presumably knows the value of what is bought. Then valuing a click involves working back from that towards a CPC bid based on the conversion rate of the traffic source.
As you say, things get interesting when value is not evenly distributed and are complicated further when incrementality is taken into account. But who said it was meant to be easy?
