CTR stands for click-through rate: clicks divided by impressions. If a hypothetical ad is shown a thousand times and clicked twenty times, its CTR is two percent. Media buyers treat it as a fast read on whether an ad resonates — a weak CTR usually means the creative, the audience, or the offer is off, and it drags every downstream metric with it, starting with CPC.
Most CTR optimization focuses on what the advertiser controls: the hook, the image, the headline, the audience. But on Facebook and Instagram there is a variable in the frame that most testing plans ignore — the comments. The top comments render directly under the creative in the feed, which makes them functionally part of the ad. Viewers treat them as social proof: evidence of what people like them think of the product before they commit to a click.
That cuts both ways. Enthusiastic customers, answered questions, and genuine tags ("this is the one I told you about") make the click feel safe. Scam links, "is this legit?" with no reply, and public complaints make it feel risky. Two ads with identical creative can post different CTRs purely on the state of their threads — which is worth remembering when a proven ad's performance sags after a spam wave hits it.
The practical implication for anyone testing ad creative: keep the comment variable controlled. Moderating threads on high-spend ads — hiding junk quickly, replying to real questions — keeps the social-proof layer working with your creative instead of against it, and keeps your A/B test results about the creative rather than about which variant the spammers found first. CTR measures the click decision; comments are part of what people weigh when they make it. Related: engagement rate, which counts the interactions that happen instead of the click.