In either case, Facebook has a financial incentive to say nothing, instead of publicizing lackluster results.
Or, my guess, they'd rather keep any news about the existence of this off the front pages, as broader knowledge of its existence by their (not as technically informed) advertising customers only negatively impacts Facebook.
Kind of how a beachfront town wouldn't want to advertise the fact that Great White sharks exist... at all.
The problem here is that adversaries are adjusting to whatever measures are put in place. In that respect it might be more like winning at Chess or Go. Which computers can do, but it’s decidedly non-trivial.
And here’s the kicker: If we postulate some hand-wavium big-data/machine-learning/AI that can detect bots and adjust when the bots evolve, why can’t we also postulate some hand-wavium big-data/machine-learning/AI that can run bots and evade the bot detection?
Then if they also happen to anyhow benefit from the existence of those fake accounts then it becomes even harder to justify costly filters and leads to a sufficient-enough-to-not-look-obviously-weak grade filtering
Facebook is entirely out of control as a social force, and PR agencies are the only thing holding back public perception.
I think what we're seeing instead is the same as counterfeits or promoted search results at Amazon: solving it could become an existential threat to either popularity or profit, so they're just "managing" it as BAU.