Before reading the article, I was thinking it was basically about some of the adult-themes on the website. I know that 'Instagram Models' are a thing on that website, and they do their work to get gigs. Some of their pictures can definitely be on magazines that are Rated 18+.
However as I read the article is seems to focus more on teenagers using Instagram and how it affects their mental state. Because of the fact that people on Instagram tend to post pictures over things, one of which being their bodies, and getting a lot of attention because of it, teenagers tend to compete and also post pictures themselves. This could either grant them the postive attention they want, or negative attention.
There's also the criticism that the social media websites don't account for minors much in regards to properly regulating what goes on. Sure they may remove a minor posting pictures that could get the website/company into a load of legal trouble for good reasons, but other than that, minors can do whatever they want on social media.
Ross Douthat’s suggestion for this is to 'create a world where social media is understood to be for adults and the biggest networks are expected to police their membership and try to keep kids under 16 or 18 out.' And what he means by this is to limit the scale of the interactions teens have on social media. I'm not sure how this would work, and given that teenagers have this 'I do what I want, regardless of the possible consequences' attitude, I don't think 'limiting the scales' of social media will help much in the problem in the long run.