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.
There is this curious word "influencer" which everyone uses but few ever think about what it really means.