The goal of Kbin is to communicate with many other ActivityPup services.
Lemmy: Link aggregator, social network, forum
Kbin: Link aggregator, social network, forum, microblogging
Mastodon: Microblogging
You can use Kbin to interact with Lemmy, Mastodon, Pleroma, Peertube, and others. That's why I switched today (and loving it, exploring everything right now).
I don't believe that there are multiple instances of Kbin right now. I only know of kbin.social (it is free and open source, though, so anyone can open another instance, I think). Edit: This was wrong. There are many instances. See the answer below.
AFAIK kbin is way early in development and got slammed by a wave of new users after Rexxit happened.
So the dev’s priority right now is server and platform stability, and it’s already way smoother than it was a couple weeks ago. That being said, there are a ton of features planned and development seems to be going pretty quickly at the moment