It's absurd to absolve a company as wealthy as facebook who optimizes for "engagement" from their externalities on children.
Is there something on the internet that isn't designed as such?
Maybe weather apps... Everything else, from wikipedia to github to stackoverflow to the site you're on now is yet another automated massively multiplayer kudo ranking system. The only meaningful difference appears to be demographic; Facebook is one of the places 'teen girls' spend their time. All I see here is evidence that clicks can still be had by ascribing some concern to the fate of young women[1].
[1]https://en.wikipedia.org/wiki/Missing_white_woman_syndrome
Gamification is benign compared to an interactive news feed developed to drive ad sales. I find pointing at especially wikipedia, but also github as being designed to maximize engagement comparable to social networks as silly. Neither has much to gain from having addicted users, the same can't be said for facebook, IG, or Reddit.
All three have various feed mechanisms. They all have various ranking systems. Every one of them have people employed (even if mere 'volunteers') to 'maximize engagement.' And every one of these systems have people obsessed with their profile. Every. Single. One.
They're just mostly not 'teen girls' and so the 'problem' makes for poor headline material.
"especially wikipedia"
Wikipedia is rife with obsessed 'editors' climbing the rungs of the interweb status ladder. They're easy to find. Look at their profile pages; filled with achievement badges (gamification) and vast profiles of their lives. Does Facebook have 'campaigns' for 'elections' that grant power on the platform? I honestly don't know because I spend no time there, but I know Wikipedia does, and we can only imagine the anxiety involved for these 'candidates.' Thankfully though, they're mostly not 'teen girls' so we're not going to worry about them.
Speaking to your point more directly, the massive, massive difference is the primary user of Instagram and typical social media vs the primary user of these other platforms. The primary user of Instagram is shown addictive feeds non-stop. It’s really not possible or enjoyable to use Instagram without interacting with designs deliberately and literally trying to get you addicted to the platform. This isn’t true for something like Wikipedia or Stack Overflow. The primary user is going to come across a few articles now and then or search something. But they are rarely going to be interacting with an algorithm or design trying to get you addicted for hours a day.
Think about it like gambling. The only way to use Instagram or Tiktok or Facebook or anything like that is by pulling the lever on a slot machine, trying to get a dopamine hit.
The inherent design and purpose of social media platforms is to addict people so they spend as much time on the platform as possible, and then show them ads. That’s not true for Wikipedia or stack overflow. Those examples are more about policies that encourage toxic behavior or just crappy people on the internet, not a critique of the inherent platform design.