What is the most important metric for social media advertising?

Posted a 7-slide carousel over the weekend. Decent reach for an account with almost no followers — 88% of it from strangers. Then I found the number that actually mattered: average slides viewed was 2.2 out of 7. I'd spent a full day building slides 5, 6 and 7. Almost nobody saw them. Views told me the hook worked. Depth told me everything after it didn't. I'd have made exactly the same mistake again if I'd only looked at the first number. What's the metric you nearly missed that changed how you build?