Systrom and Krieger built Instagram the product, Zuckerberg built Instagram the business
Zuckerberg tends to avoid monetisation, leaving that to Sales (i.e. Sheryl) and Ads.
Sheryl is not in to sales. And, zuck is all about business but he don't think about monetisation until that product have at least a billion users [1].
The real person behind instagram's monetisation success - https://finance.yahoo.com/news/meet-man-helped-facebook-brin...
[1] - https://www.businessinsider.com/zuckerberg-products-need-1-b...
I'm aware that Mark (he doesn't like zuck anymore, apparently) thinks that, but he focuses on product much much much much more than monetisation (there's an apocryphal story that suggests he said that ads weren't part of his vision for FB to an internal sales conference in about 2009-10).
Finally, I would credit the engineers in Ads a whole lot more than Mr Weil (to my recollection, all of the stuff that ended up making IG money was in place before he joined).
Source: worked for FB for a long time.