The net value of Facebook ads would roughly be taking some measure of how much it helps minus some measure of how much it hurts.
You posted some way of roughly approximating how much it helps. But looking at metrics like this in isolation is meaningless and there's no useful interpretation except for people (such as in the ad business) whose utility function for this topic only depends on the upside component.
Society at large also depends on downside components, like how much psychological drag the pervasive ads generate, how many people are deceived or manipulated into mis-wanting something purely because of ad repetition, how many times are ML-based personalized ads miscalculated, leaving a consumer to experience inappropriate or irrelevant ads because of algorithm malfunction. How much extra cognitive effort is required to maintain browsing solutions that mitigate tracking. How much of the ad experience facilitates addiction to mobile phones. And so on.
I am not claiming that I have a great way to roughly quantify these, but you could. For example, you could do eye-tracking and phone usage studies and try to roughly compute the amount of extra app time that is attributable to dealing with unwanted ads or app time attributable to ad-linked addictive behavior, and then estimate up what some average case cost of that time would be at a median salary or something, to roughly value that time. Or you could gather statistics on how accurate data-based ad personalization is and how often someone spends time costs processing misclassified ad categories they self-report as not relevant.
I conjecture that the financial cost of that lost time (which is an under-estimate that doesn't factor in other psychological drag or second order effects) would, when aggregated over hundreds of millions or billions of people, add up to waaay more cost that what the offsetting value from effective ads and product discoveries offered, meaning that from a perspective other than the consumer corporations that profit from the transactions, it would look like a giant net loss for society.