CPC stands for cost per click: total ad spend divided by the number of clicks. If a hypothetical campaign spends $100 and gets 50 clicks, its CPC is $2. It is one of the oldest metrics in digital advertising, and on Meta it is really the ratio of two other numbers — what you pay per thousand impressions (CPM) and how often those impressions turn into clicks (CTR). Cheaper impressions or a higher click rate both pull CPC down.
That decomposition is what makes the comment section relevant. Comments do not change what an impression costs directly, but they absolutely change how often an impression becomes a click. People read comments before they click — it is the closest thing an ad has to a review section. A thread where real questions get real answers gives a hesitant viewer the last push; a thread of scam links and pile-ons gives them a reason to keep scrolling. Same creative, same audience, different CPC.
There is also a subtler leak: on Facebook and Instagram, the click you pay for is not always the click you wanted. Engagement-optimized campaigns can rack up clicks into the comment thread itself rather than through to your site. A chaotic, argumentative thread attracts exactly this kind of rubbernecking traffic — engagement that looks like interest but never reaches your funnel.
The moderation play is to make the thread work for the click instead of against it: hide the junk fast, and answer the "does it come in blue?" and "how long is shipping?" comments publicly, because every answered question de-risks the click for the silent majority reading along. The commenters themselves are often the warmest prospects on the ad — which is why a buyer-intent comment deserves a faster response than any other. CPC tells you what a click costs; the thread under your ad quietly decides how many you get.