Scratch further and beneath the ad business you'll find more incentives to allow fake engagement. Man is a simple animal and likes to see numbers go up. Internet folklore says the Reddit founders used multiple accounts to get their platform going at the start? If they did, they didn't do that with ad fraud in mind. The incentives are plenty and from the people running the platform to the users to the investors - everyone likes to be fooled. Take the money out and you still have reasons to turn a blind eye to it.

The biggest problem I see is that the Internet has become a brainwashing machine, and even if you have someone running the platform with the integrity of a saint, if the platform can influence public opinion, it's probably impossible to tell how many real users there actually are.