CPM stands for cost per mille — the cost of one thousand ad impressions. If a hypothetical ad set spends $200 to serve 20,000 impressions, its CPM is $10. On Facebook and Instagram you do not usually bid a CPM directly; the auction produces it as an output, shaped by your audience, placement, time of year, and — the part that gets overlooked — how people respond to the ad itself.
That last input is where the comment section enters. Meta's auction does not just sell impressions to the highest bidder; it estimates how good an experience each ad will be for the person seeing it. Ads that people hide, report, or react badly to are judged worse experiences, and the auction effectively charges more to keep delivering them. Those signals are summarized in Meta's own diagnostics as quality ranking and negative feedback — and a comment thread full of scams, arguments, and unanswered complaints is exactly the kind of experience that generates them.
CPM is also only half of any efficiency story. It tells you what attention costs, not what attention earns; a low CPM on impressions that never convert is money spent quietly. That is why media buyers read it alongside cost per result and ROAS rather than in isolation.
For comment moderation, the takeaway is simple: you cannot set your CPM, but you can stop making it worse. Keeping the thread under a high-spend ad clean removes one of the negative-experience signals the auction prices in, and it protects the value of every thousand impressions you buy — because each of those impressions renders your comments right along with your creative.