I think the alternative would not be that teenagers would not have social media accounts, but that they would all be on Tiktok (which probably is already the case)
TikTok is a much more positive platform than anything Facebook has come up with TBH. Almost all of the creators I follow are just... normal people, making normal jokes and doing normal things. They aren't heavily airbrushed or photo/videoshopped or whatever. The content itself is generally much more positive and much more focused around acceptance and (body-)positivity. As far as social media platforms go, TikTok is much "healthier" than Instagram.
Maybe because the TikTok algorithm is feeding you what you value. Whereas for people with different values, maybe people with certain insecurities that are in the process of change (mainly adolescents) they might be drawn to people that have what they lack just like any other social media platform.
TikTok is incredible, in my opinion, it really is the best content-delivery platform... but that is actually quite dangerous because what people want to see is not necessarily what they should see. It's important for people to have broad perspectives and I don't think TikTok addresses that so appropriately.
TikTok feeding us what we apparently value is its biggest danger.
When people out of mere curiosity watch something biased/misrepresented/discriminatory/extreme and interact with that content, irrespective of whether all their opinions at that moment of time are the same as that of the content they watched, the algorithm will aggressively push similar content onto you, and there will definitely be a few who fall into "the doom".
The former, which is why people use it. It shows you what you want to see and will genuinely enjoy seeing.
That is why TikTok is more popular than platforms that only focus on engagement, because those platforms will show content that you will engage with even if it's in frustration.
So if I want to see you naked, TikTok will show me it?
It's disappointing to see folks in hacker news regurgitate TikTok marketing material as fact credulously. No service could show you precisely what you want to see. TikTok does it's best - but e.g. if I want to see no staged video, TikTok can't enforce this, because TikTok doesn't know.
TikTok is not an adversarial platform and there is probably an element of East vs West styles of thinking here. TikTok's engagement is driven up by people taking other's videos and building on top of it. FaceBook's engagement is driven up by people taking another's position and arguing against it. Leads to very different styles of engagement. You can find toxic content on TikTok as well and build on top of it but it is harder and you have to seek it. On Facebook, you could make a slightly partisan post and someone will take an opposite position and then you harden your position to defend against that and others join in until it turns into a pissing match.