The way I have it, Mastodon works like Twitter. Kbin, Lemmy, etc. work like reddit, as a way to gather together tons of individual forums/subforums.
Only, because they all use the same networking protocol, they're all compatible with each other. So imagine being able to use your reddit account to follow/respond to a tweet. The whole thing comes off as so much chaos to the uninitiated, and I'm kind of liking it