What they (Facebook) are doing now seems to be some version of DADT (Don't Ask, Don't Tell).
Does me pointing out the barely shrouded subtext mean that I believe what they are saying? Of course not. It's just as likely, it seems, that their test is simply flawed (as others said, let's see the technical honey trap), that it's some other technical fault, etc. I highly doubt, just on intuition, that Facebook is running such broken bots: Come on, they'd do a better job than make something so easily detected.
That to me sounds like FB is aware of the issue and isn't interested in discussing it. That's the subtext imo.
My own suspicion would be because someone is gaming their ads and they haven't been able to prevent it as of yet, and not because they themselves are gaming the numbers. No doubt they believe that discussing it is tacit admission that they haven't been able to prevent it, which is bad for business.
And it doesn't matter who it is, because they're leaving for a different reason: Facebook never returns their calls unless it's with an offer of extortion.
Advertisers monitor to see performance on the other end, and when the campaigns don't yield actual success they move on.
The only real exception to this are mega-brands that advertise just to build namespace. Everyone else expects direct action from some hefty percentage of their ads.
Imagine a 100 person company with 5 people dedicated to advertising. Leadership says to increase on-line presence. Some recent college grad on the advertising team buys FB ads. Next month leadership is pleased that they're advertising on Facebook. Also leadership is pleased that sales increased x%. The FB ads could be completely worthless. Some non-trivial percentage of clicks could be from bots and no one would notice.
Most ads target a specific outcome: You buy a product, create an account, joining a mailing list, etc. Even the most technically incompetent advertisers establishes these fundamental metrics such that they can say "I spent $1,000,000 on Facebook ads and only got ten signups. I spent $1,000,000 on iAds ads and got ten million signups." That sort of thing.
Assuming the problem is as bad as many are saying, I personally think it's fair to draw the same conclusion you have -- running and letting run a botnet is one and the same. It's costing other businesses money; it's too big for Facebook to be able to play dumb on the issue.