> Instagram just shows me all the stuff my friends post. That's it, as far as I can tell.

Unfortunately, Instagram is not just "showing you everything your friends post".

I’m kind of curious as to what the person is seeing, because every third post I see is an ad placed by Instagram, and for some of the meme pages you reach nearly that much saturation with sponsored posts.

I mean obviously there are ads, but if my friends post something, it's at the top of my feed, and I can scroll through only content that my friends post (and ads).

It's still not just a linear feed, though. Years ago it was just a reverse-chronological feed of things your friends post. Now the ordering is "algorithmic". I can't think of any good reason to reorder posts other than to try to increase engagement, which is the problem we're talking about here.

If you don't allow curation, that's the end of moderated forums like Hacker News, isn't it?

Who said anything about not allowing curation?

> make them responsible for promoting or curating the content, if they choose to do so

So, Y Combinator can choose to be liable for anything defamatory that manages to get onto the front page (there is, after all, an algorithm), or anything defamatory that they fail to delete out of the comment section. It doesn't seem sustainable.