Cost per result is Meta's umbrella efficiency metric: total spend divided by the number of results, where "result" is whatever your campaign is optimized for — purchases, leads, link clicks, messages started. If a hypothetical campaign spends $500 and produces 50 leads, its cost per result is $10 per lead. Because it adapts to the objective, it is the number most media buyers scan first when deciding what to scale and what to kill.
Two forces set your cost per result: what you pay for attention, and how much of that attention converts. Most optimization effort goes into the first (bids, audiences, creative testing). The comment section quietly works on the second. Comments are attached to the ad itself, so every impression you buy renders your creative and the public conversation under it. A thread full of scam links and unanswered complaints makes the same impression convert less often — which means you need more impressions per result, which is another way of saying your cost per result goes up.
The auction compounds this. Meta's delivery system weighs how people react to your ad, and signals like reported comments-section toxicity, hidden ads, and poor engagement quality feed into quality ranking and negative feedback. An ad that people react badly to tends to become more expensive to deliver — so the damage shows up twice: fewer conversions per impression, and pricier impressions.
Comment moderation is one of the few levers that works on this without touching your media plan: hide the junk, answer the questions, and the impressions you already committed to buying do more work. It complements creative and audience optimization rather than replacing them — and it protects the top-line metric behind cost per result, ROAS. For the mechanics, see reducing cost per result with comment moderation.