Runtime is the signal. Engagement is not.
The most useful thing a competitor's ad library tells you is how long an ad has been running. Nothing else visible from the outside correlates as closely with whether it works.
You cannot see a competitor's cost per acquisition. You cannot see their return on ad spend, their creative testing budget, or the meeting where they decided to kill a campaign. From the outside, almost everything that matters is invisible.
One thing is not. Public ad libraries show when an ad started running and whether it is still running. That single pair of dates is the closest thing to a performance signal available to anyone who is not inside the account.
Why longevity carries information
Advertisers turn off ads that lose money. Not always quickly, and not always rationally, but over a long enough window the pressure is one-directional. An ad that has been live for four months has survived every budget review in those four months. An ad that ran for nine days did not.
That is a weak signal on any single ad. A brand might leave a loser running out of inattention, or kill a winner because a campaign ended. Across a whole category, the noise largely cancels and the pattern does not.
Why engagement metrics mislead
Likes, comments and shares are visible on some platforms, which is exactly what makes them tempting. They are also close to useless for judging a direct-response ad.
- Engagement measures reaction, not purchase. The most-commented ad in a category is often the most divisive one.
- It accumulates with spend. A mediocre ad with a large budget outscores a strong ad with a small one.
- It is trivially inflated, and in some categories routinely is.
A long-running ad with no visible engagement is a better bet than a viral one that disappeared after a fortnight.
What to do with it
Rank every ad in your category by how long it has been live. Read the top of that list closely: the format, the opening three seconds, the offer, what the first line of copy asks the viewer to believe. Those are the arguments that have survived contact with a real audience and a real budget.
The oldest ad in your category is the one your competitor has been unable to beat. Start there.
Then look at what is missing from that list. The angles nobody has run for long are either bad ideas or unclaimed ground, and telling those apart is most of the work.