(assuming your comment wasn't sarcastic - maybe it was?)
I've never understood this. Shouldn't the metric for how well a promotional post performs be... actual sales (or at the very least, traffic to whatever you're promoting)?
Granted, likes might be a useful proxy metric for (roughly) gauging interest in something, but are you really making important business decisions based on a post's likes? What are you comparing those numbers to, and how do you know what they mean in terms of real business impact? How do you know people are responding to the product, and not just the model (or "influencer") you happened to use in your promotional image?