It's funny because most of the commenters found the song because of this mountain coaster video. 댓글을 다는 사람들 대부분이 "mountain coaster" 비디오 때문에 그 노래를 알게 됐다는 게 재미있어요. comment = 댓글 to comment = 댓글을 달다 most of the people who comment = 댓글을 단 사람들 대부분

참고로 말씀드리면 "주석"은 인터넷 상의 comment 라는 의미로는 전혀 쓰이지 않습니다. 주석은 텍스트(특히 논문이나 학술서적적)의 annotation(주해) 또는 footnote(각주) 라는 뜻으로 인터넷의 댓글과는 전혀 다른 말입니다.

Sorry for the confusion. I really should have included context. I saw a video of a "mountain coaster" (like a luge ride) in the Swiss Alps. The video was accompanied by a song that most people have never heard. When you look up that song online, say on YouTube, you'll find that most of the people who are commenting about that song found it by first seeing the mountain coaster video. - I hope that makes more sense. In any case, thanks for the answer. I wasn't sure how to say "(post a) comment".

It's a bit hard to answere, 'cause honestly even I couldn't guess what's the meaning of "found the song because of mountain coaster video"... Anyway, I'm gonna try. First, I'm sure your writing make sense and it's right. And I think it would be better to replace '주석' with other word. Actually, I'm not sure what you meant, I think you was trying to say comments people post in website like italki. It would be much better to use '댓글'. It's more specific term and only used in web. But '주석' is barely used in practical conversation or writing, it's likely to be used on thesis written by high educated old person. And you'd rather use verb '달다' or '작성하다' or '쓰다' than '하다', when you use '주석' and '댓글'. '작성하다' and '쓰다' mean 'write' and '달다' means 'post' or 'hang'. So it would be like this "대부분의 댓글(주석)을 작성하는(다는 or 쓰는) 사람들". And you can say "댓글(주석)을 작성하는(다는 or 쓰는) 사람들의 대부분".