I'm using upvoting and downvoting in a generic sense. I'm using “upvoting” to mean taking any action that boosts content, that spreads it, and particularly that makes it seem more relevant to the algorithm, and “downvoting” to mean anything that does the opposite, that limits the reach or makes it seem less relevant.
Each platform does that in a different way. An easy example would be “liking” something on Facebook, which is a way of upvoting it, because in future,things that you “like” will be seen as more relevant to you. Facebook is more likely to show you that if you've selected “most relevant” rather than “most recent”. You do have the option on Facebook of toggling to just a straight timeline, but the default normally is to be shown what the algorithm feels is relevant to you.
The reddit platform, for example, has pure upvoting and downvoting. In reddit, each user can literally boost something by making it more popular or drop it by making it less popular. In the case of reddit, that's also a big issue in terms of what appears on the front page of the site, which is to say what you see when you just go to reddit.com, rather than one of the many sub-reddits. That is something that we know hate groups have manipulated. They have made an effort to get certain hateful messages to the home page by getting enough people to upvote them, and again, when they've decided to target particular critics, they have tactically downvoted them in the same way.