You can interact with Mastodon (a microblogging site like Twitter) via the microblog tab. By going to the All page and then clicking Microblog, you'll see everything that Mastodon sites share with kbin.social (and vice versa).
Individual Kbin magazines work a bit differently. A magazine's microblog tab only contains:
- what people on Kbin post to that magazine's microblog tab
- any microblogs (e.g., Mastodon posts) that have certain tags
Each magazine can set which tags its microblog tab will collect. For example, @pixelart has microblogs with the #pixelart tag. This is nice if you only want microblogs on a specific topic, but if you want to browse everything that Mastodon shares with Kbin, you'll need to use the All page (and make sure that you have federation turned on).
EDIT: Reworded to be a bit clearer