I'm not sure if that's true anymore; I read the other day that e.g. Youtube's algorithm works with engagement, that is, the sum of up- and downvotes. Because downvotes don't mean it's a bad video anymore, it means that people disagree with it - but still would watch it. The algorithm probably also takes views, time watched (until voted), total time watched etc into consideration though. And some magic sauce depending on how advertiser friendly it is, which likely explains why the default suggestions (in anonymous browsing mode) are so weird.