I don't have kids but hope to one day. I've thought a bit about how I'd approach similar scenarios. Take my answer with a grain of salt, as it's probably not one you're going to like.
> If you had to let your kid have a social media account, which network would you feel least concerned about, and why?
All of them are equally as harmful or beneficial as the user makes them. If my child were to allow others' opinions blindly sway them, or if they easily let their emotions get the best of them, then no social network is going to do them any good.
Instead, I would be more concerned about their ability to reason about harsh comments, considering differing opinions, and overall having good critical thinking. After that, it doesn't matter what someone says to them on the internet - they'd be able to have a more thoughtful, mature understanding of a given situation and be able to respond more maturely and appropriately.
Isn't this what it's about, anyway? Sheltering them from nasty stuff isn't really preparing them for the world. The world is a very nasty place. If they aren't secure in their reasoning and emotional control while sitting, safely, in their own home behind a computer/phone screen, then they're really going to have a tough time out in the real world.
> Note that although "alternative" social networks are interesting
This is how you get your kid indoctrinated into anti-semitism or other ideologies. Most "alternative" social networks these days exist as newfound homes for extremist or inhumane groups. There are exceptions, as I'm sure someone upset about my comment will point out, but you're doing what we call in the industry a "bandaid patch" by forcing them to such networks.
> I'm mainly interested in your thoughts on the main social networks that teens use, like Instagram, Snapchat, TikTok, etc.
All of them spy on you, use your data, do facial recognition, and prey on their users, regardless of age or demographic. As will any alternatives, regardless of what they claim.
As for the "social" effect, it really depends on who they follow/interact with. No platform is going to be inherently better than the other in that regard.
> my teen doesn't seem to care
Because your teen is probably optimizing for something different than you are. They're in a different life stage, after all.
They haven't been through the life experiences to really value privacy yet, as they haven't seen the direct consequences of it being taken away, or had that "wtf" moment when they see private moments shared to the public in one form or another.
This is all part of the learning process. A teen should be a teen, not an advocate. You can educate them all you want but ultimately it needs to be their own choice. The people I've encountered that are the least prepared for life tend to be (though are not exclusively) those who had very sheltering/protective parents.
Being a teen is all about doing things yourself and failing. You don't have to worry about the choking hazards anymore, so let them break some stuff and deal with the results - even if that means ego/reputation/etc being hurt in the process. That's part of growing up.
Just my $0.02.