TikTok's algorithm is based entirely on when you click the Like button and when you linger on a video, exactly like StumbleUpon's algorithm. StumbleUpon even had a video product, StumbleVideo, that was basically just TikTok.
But, in 2018, when StumbleUpon shut down and sold their assets to Mix, the prevailing wisdom was that people didn't want to use StumbleUpon because they wanted to use Reddit and Facebook, to follow curated feeds of links, instead of random links that other people like.
If that wisdom were true, TikTok should have failed too, because TikTok just gives you "random stuff that similar people like," just like StumbleUpon.
I guess it just goes to show that there's no accounting for the rise and fall of social media apps/networks.
(Maybe it's the swiping gesture? Maybe the gesture is more comfortable in portrait?? But it's hard to see why that would make or break a video app like this…)