Here's my writeup on why TikTok is doing so well: https://www.danielxli.com/posts/why-im-obsessed-with-tiktok
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I think they also nailed the music aspect of content creation. Whereas other platforms are actively hostile to users using unlicensed audio, TT licenses popular music which saves a boatload of effort on the content creation side and just makes it all more compelling to watch. Contrast that with takedown notices and little support for including sound at all.