For me the majority isn't even relevant, I'm on Twitter specifically to follow certain people or organisations which are related to my interests. I barely use Twitter these days but most of the accounts I follow are still on Twitter and bascially nobody is active on Mastodon (or any other alternative I can access for that matter).
I love the idea of Mastodon, but it's worthless to me if my timeline is empty. Lemmy and Kbin are different because they're organised around topics instead of people.
It depends on how it's implemented and I've never used the reddit API, but I assume it's just a single API call every time new posts are loaded. So it'll load a batch of posts and then once you've scrolled down far enough it'll make another call to load more. But basically everything you do in the app that interacts with reddit causes an API call, e.g. open a post/load comments, upvote, post or comment, view a profile or subscribe to a subreddit. Depending on how the API is designed, multiple calls may be needed.
Twitter alternatives for the Musk-averse ( www.theverge.com )
Twitter isn’t the only social network around — here are some others.
Reddit's updated rate limits going into effect over the coming weeks, not entirely on July 1st, as was expected ( www.reddit.com )
This is why apps like Boost, who never signed up for a subscription model are still functioning....