I dunno if it’s exclusively his innovation, but Zuckerberg’s insight was that you could actually lie to your users by presenting yourself as trustworthy, while having malicious intent to violate their privacy by selling their data to advertisers, and there are no business repercussions for doing so.
Instagram ads are the most consistently interesting ads I see online, and by an enormous margin. Ads suck, and I resent that their inherent purpose is to manipulate me, but they accomplish their goal better than any other ads I've ever seen.
I don't mean it in a "super bowl commercial" way; the products in Instagram ads are the consistently interesting part. It seems like a safe assumption that the advertisers for plenty of uninteresting products are trying to buy ads on Instagram, and that the advertisers for plenty of interesting products are trying to buy ads elsewhere. That means that Instagram's targeting is the differentiating factor.
I guess my argument is that it's not innovative or "hard" to sell ads to people who have given you massive amounts of personal and demographic information (voluntarily!)
Something something even Apple wants 10s of billion$ a year from it… building the big money machine is an innovation of a kind when only a few entities can really do it.