Lemmy, kbin and mastodon instances can interact with each other via activitypub. People can migrate to literally any instance they want, then subscribe to any community they want regardless their origin instance.

My mind was blown the first time I watch this this interoperability happen. If twitter, reddit and hackernews support activitypub, it would goes like this: if someone post a tweet on twitter that mention a subreddit in the tweet, then that tweet is automatically visible in the mentioned subreddit as a post. The people on that subreddit can comment on the post, and it'll show up in the original author's tweet as reply to the tweet. Then in the middle of it, hackernews user somehow join the discussion because someone on the hackernews subscribe to the subreddit.