Are you certain? Medium has started putting blogs behind their paywall by default, and authors are generally unaware of that. Most recently, I had to notify Google Design to remove the paywall.

They only talk about Instagram in the article. It’s an ad for the service they run, so there’s not much info besides “this horrible thing happens regularly, we’re working on it.”

> As a parent of 9 and 7 year old girls, I'd like to know what platforms to be weary of.

All of them.

Honestly you should be worried about all platforms. This article applies to abuse on Instagram but this is not the only problem and not single platform. If it comes to Instagram IT was the first place where i noticed how dangerous social media can be. It was not so popular like now and it was not "as safe" as now. Some years ago i have seen a lots of posts with photos of people cutting hands. It was extremly disturbing. My advice is to educate and keep away children from social media as much as possible. This is a tip of an iceberg how it can destroy young lives