I am new and signed up using kbin cafe but seem to be able to post in kbin social. I am guessing each website is like its own email service from what I can see. Though I am not sure if the owner of kbin social is the same as the owner of kbin cafe. I take it anyone can set up their own kbin site and connect it to the other kbin sites?
You can also connect to non-kbin sites, including Lemmy and Mastodon! They all use the same protocol to be able to talk to each other.