If it was only network effect, then how did TikTok grow in a space where Instagram and Youtube were already much bigger players? How did they gain that user base?

Network effect helps, but it only explains why they stay big, not how they got big

There really isn't that much making TikTok unique. Yes, their app is well designed. Yes, stitches and video replies make for great social/parasocial features because creators are actually interacting with each other and the community, almost like tumblr. But in my opinion those are reasons number three and two why TikTok is successful. Their recommendation algorithm is number one, by a wide margin.